Historically one of the traits of an Eldar army is the emphasis on synergy. At the time of the Harlequin release the prevailing theory was that a redone Eldar book would compliment the Harlequin book. In some ways this was very wrong.
With the ability to reflect upon the compatibility of the two new books we can see that the big loser is the Cast of Players. At the time of release it was a way of nearly guaranteeing a substantial run roll for Eldar and Dark Eldar, considering two dice with rerolls give exceptional odds for a quality roll (nearly 80% chance you'll get a 5 or 6). Unmatched Agility makes the bonus to Eldar all but moot.
This tells me The Cast was likely a throw-a-way formation to carry over players that already had a unit painted and ready for play. The real bonus comes from larger formations, but to play this means you're probably using Harlequins as a whole force, or using them as a section of an apocalypse force in a mega battle. They operate on their own in the theatre of war, and have one of the better special rules a formation gives, specifically Rising Crescendo.
All things considered the Harlequins are a tremendously effective boost for Dark Eldar, making some some of their units just that much more effective at crossing the table reliably.
That said I would not be surprised if The Cast became a formation that saw common use in tournaments that allow two or three formations, partially because of the troll aspects o the Harlequin Warlord traits and partially because in the hands of a good player they can bring an assault element that most opponents aren't ready for.
Valedor, while less than a year old, now feels heavily outmoded by Harlequins and Craftworld Eldar. More strikingly it stands out as a missed opportunity to have an umbrella formation that includes the three branches of the Eldar faction. Now, more than before, the Dark Eldar book feels like a false start, with only a single, very restictive detachment included. It also currently appears to be the end of supplements, considering no book released since is a supplement, instead what would previously have been a supplement is released as a stand alone book. I'm curious to see what happens going forward, and how it inspires me to adapt my Ulthwe war host.