Well, with a few months of play under my belt I feel I've got a good idea of the differences between the prior book and this one. There are a substantial volume of minor tweaks to units, but the biggest difference (which I get to at the end) is the addition of formations and encouraging tank squadrons. I gave a short blurb for pretty much each unit, but I didn't go too in depth as overall things didn't change too too much.
Exarch powers – they've been cleaned up in that now they're not optional, and this means people aren't ignoring half the options. It also means they've been dropped from the Avatar.
Psychic powers – In my opinion this is now the best psychic tree in the game. The replacement for Death Mission (Will of Asuryen) is a very potent power capable of providing synapse style support to leaders with questionable leadership values and Eldritch Storm has finally been brought to where I've wanted it to be for ages, a true end boss style super power. Combined with other powers like the divination power that allows you to ignore cover or guide it's incredibly potent. Moreso when it's cast the shockingly high WC 4 version. Even Mind War was improved to remove the psychic backlash aspect of it.
Avatar – Now LoW
Autarch – Really there wasn't much in the way of change, but some of the wargear did shift, the banshee mask in particular now allows the Autarch to provide a real support role to the use of aggressive disruption tactics to units like Swooping Hawks. Ultimately if you used this guy before you will continue to use it, but he can still drag the best abilities from certain units like banshees, hawks, and scorpions.
Farseer – Pretty much the only change was that the runes were made incredibly good, and built in to the unchanged base cost. Point for point it's the best psyker in the game.
Spiritseer – Spirit Mark is now better. Not much else has changed, but I no longer forget to pick a unit to spirit mark so I get less aggravated when I use them.
Warlock Conclave (Formerly Warlock Council) – Completely reworked as a unit. They are no longer pilfered to give warlocks to guardian squads. In addition they made them a brotherhood of psykers, limiting the bookkeeping for large units (and limiting your ability to tap the well to cherry pick the powers you really want), but allowed them to keep a mastery level for each warlock which meant they remain viable at their standard cost. I'll save discussion of this unit for the formations.
Howling Banshees – Wow. What a difference a few changes can make. They're faster, cheaper, and their masks do what we wish they did in the last book. Jain Zar makes the unit scary fast and adds a hack of a punch as well.
Striking Scorpions – Mandiblasters are better and Shadowstrike was added on. A beastly unit now and Karandras provides some major boosts as well. The Exarch power makes for a nice little bonus in most combats and synergizes very well with Enervate or Enhance as the case may be.
Fire Dragons – They got a major buff against vehicles wrecking them on a 4+. I'm not sure why they got that other than to keep them an option in the face of new wraith guard.
Harlequins – Now in their own book.
Wraith units – Previously I wrote about these units in TROOPS, but now the ability to bounce units around the FOC is gone.
Wraithguard – I'm pretty sure everyone knows just how fierce the guard are for both original and scythe versions. Even moreso than Fire Dragons these units are pretty much point and kill.
Wraithblades – They are unchanged from the prior book, so their use is exactly the same, just rush forward and hope enough survive to actually take out their intended target.
Guardian Defenders – Other than the Warlock being built in to the unit there is no change.
Storm Guardians – As above
Windriders – The big change here is that all bikes in the unit can upgrade to a heavy weapon, either shuriken cannon, or now scatter laser. People in tourney circuits love all scatter lasers, but I find that makes units too pricey and unable to handle casualties. I personally limit myself to at most 1 per 2 bikes, and I stick with cannons because I find their versatility is worth the lost shot and range.
Dire Avengers – Deference Tactics is a really great bonus to the unit and gives them added flexibility when it comes to holding objectives.
Rangers – They've upgraded from stealth to shrouded, but their overall use is the same.
Wave Serpent – The shield was fixed. Lower strength, one use only, range has been reduced by more than 2/3's, and pinning is replaced with Strikedown. For once everyone seemed happy with rules change to a unit despite it being a hefty nerf. Now the Serpent is actually a transport more than a gunship and faux anti-aircraft fire platform.
Vyper – 10 pts Cheaper, and considering I always used a squadron of 3 with dual cannons I can now have that build for 30 pts cheaper. People don't give the unit enough respect, which, if you use them aggressively can be played to your advantage as they tend not to take fire until after they've done their damage.
Swooping Hawks – 6” of extra move has made them even more impressive. They've also been gifted the ability to give a vector strike style haywire attack to flyers and flying monstrous creatures. They are scary good. Baharoth can make them faster and more potent for disruption.
Warp Spiders – Their movement was changed and now when warp jumping ignore dangerous terrain checks. They also got the flickerjump rule pegged on making them potentially more survivable, but really it just makes them more characterful. Monofilament was toned down, no longer providing bonus strength, a notable nerf in regards to firing at vehicles, but that's not such a bad thing, really. Monofilament rolling against Initiative harkens back to second edition and will be a point of terror against things like the riptide, now getting wounded on a 2+ and Wraithknight, suddenly getting wounded on a 3+. It also means Warp Spiders are less potent against Slaaneshi daemons and the Avatar, unable to wound models with an initiative 10.
Hemlock – D-scythes got a boost (though I still wish they ignored cover) and it went up to ML 2 to be in line with the standard Spiritseer. It no longer has fixed powers, giving it variable roles. The change to the Mind Shock pod is alright, but is situational. Unfortunately it doesn't synergize with Psychic Shriek.
Crimson Hunter – The discount makes them more appealing with a vast array of competitors within the slot.
Shining Spears – They gained a rule that gives them a 4+ cover save just for moving, sort of recreating where they were in 6th with the classic Jink, but they lost access to hit and run, close to a death knell for the unit, especially since they still have the old models. They're best off coming on as reserves and then going directly after priority targets.
Wraithlord – Previously I felt this unit should have been moved to elites like dreadnoughts and riptides and I still think that, but it's less of an issue now. For its points it is a solid choice, but it doesn't really stand out.
Wraithknight – Now a LoW.
Support Batteries. The Vibro Cannons are unchanged. Shadow weavers got the new monofilament rule, so the big news are the D-cannons getting D strength. Their use hasn't actually changed from prior iteration, so if you used them before you can still use them in exactly the same manner, menacing area denial.
Fire Prisms – Mostly unchanged, but now they can be taken in squadrons, and when doing so have the option of combining fire similarly to the 4th edition book. Sit back and fire using range as your best defense.
Night Spinners – Similar to Fire Prisms they gained the ability to squadron and get real synergy from doing so by having their shots get stronger the more are added in the unit, providing a very potent barrage.
Falcons – Like the other tanks in the slot they can now be taken in squadrons, but doing so doesn't provide any bonus until you have 3, though what a bonus it is, gaining deep strike with no scatter. An entire battle plan can built around this unit and their occupants since they're so reliable, but you may want to consider Autarch support to reduce the chance of them not arriving when you need them.
War Walkers – Virtually unchanged from the prior book, coming with scout and battle focus and a 5++, so the only real difference here is that they no longer need to pay an additional 10 pts per EML to get skyfire missile access. If ever you tried them out with 6 EMLs it saves you 60 pts. Against Tyranids using flying MCs you can see this being a handy (though pricey) option.
Dark Reapers – still great, but now 5 pts cheaper per model and gained inescapable accuracy. Paired with their reaper rangefinder helmets they can be an absolute terror to some units.
Lords of War
WraithKnight – it's finally a GC, has a D str attack with the sword and got a price bump... though not as much as I was expecting. I'd consider it about 50 pts too cheap, but it's not the only under priced unit in the game so I don't stress too much about it. But, similar to the Imperial Knight, it can take over a game.
The Avatar – It lacks the ability to be modified by Exarch powers, but it gained a furious charge and rage radius buffer, bonuses that effect itself, and gained +2 str in CC thanks to the wailing doom. It's a fantastic support unit, and capable on its own of carving a swath through the enemy. When you have the Avatar try build the list around it.
The new area to look over and probably the biggest area of change in the book.
Craftworld Warhost – Amazing. Any unit with battle focus runs at 6” guaranteed. This makes your units dependably terrifying and only requires one guardian style build along with one auxiliary unit.
Guardian /Storm Guardian/ Windrider Battlehost – On their own they're meh. The free heavy weapons are nice, but really you're probably only taking one of these if you want to unlock the Craftworld Warhost.
Seer council – Amazing. This unit is expensive though, so be aware a full size squad sporting Eldrad Ulthran is effectively a DeathStar by design and will run at Scout titan costs. A lot of the time this unit will control the game and the way you use it will dictate whether you win or lose. Capturing warp charge on a 3+ doesn't just make your casting more reliable... it means you're throwing fewer dice to cast powers and therefore are casting more powers overall. Add in the fact that the Warlock Coven works the way other armies wish their psychic brotherhoods worked by manifesting a warp charge despite not acting as a separate psyker means your bookkeeping is reduced and you can focus where you want to roll your powers based on what your Farseers rolled up. As a foot unit they are totally viable, and if you really want you can just buy a Wave Serpent as a FA choice and load them up to give some protection if you don't get the first turn.
The Aspect Host – Another winner. The ability to choose between BS and WS isn't even a real decision most of the time. BS is far more useful, even for hand to hand units. +1 WS will require a hand to hand style build paired with opponents that have a comparable weapon skill. That said the +1 BS is absolute gold and can ignite a real nightmare scenario for opponents. That doesn't even account for the rerolls to LD tests that are tagged on to these units.
Dire Avenger Shrine – An alright choice, but nearly unnecessary in comparison to the Aspect Host. The somewhat return of the old BladeStorm is nice if you really want to do a sweeping alpha strike whilst deploying from Wave Serpents, but considering two of the units can't upgrade to have an Exarch this particular formation doesn't have much shine for me. Perhaps if it also gave Objective Secured I'd be more enticed.
Crimson Death – A very solid formation that can transition these units to something much more survivable. That said it is an expensive formation for 3 units that are still very vulnerable to mild firepower and often have their firepower reduced drastically.
Wraith Host – It bumps up the Spiritseer's ability range and makes it more potent, and gives Battle Focus to all the units in the list. For units that are normally ponderous and with short range it's a stunning upgrade. As part of a Craftworld Warhost it's stupidly good and can catch many opponents off guard if they're not used to that speed coming from that footslogging force.