It took more than a year, but I finally finished:
When I first painted a single squad of Harlequins I tried as much as possible to make them all individuals. With the release of the new book I spent a while contemplating how I'd like to use them going forward and what possible masque would be best paired with Ulthwe. There was one little mention of Midnight Sorrow coming to Utlhwe's aid early in their big reveal, but no real notes since. Finally Death Masque came out, with a new plastic Eldrad and cementing a tenuous bond between Ulthwe and the Midnight Sorrow, so my decision was made. While I haven't gotten around to painting the Death Masque Harlequins yet I have painted all the ones I'd previously bought and built. 1750 worth of Harlequins in a full Cegorach's Revenge formation. In the years since I've first painted them my tremble has made the diamond pattern a bit more difficult, but I approached it as best I could.
I've even gotten to run the formation in a game so I can start understanding them a bit more. My thoughts on that below:
I played in a game against Minotaurs with a unit of Custodes acting as a bodyguard to their Chapter Master, supported by a Leviathan dread. My thoughts are, super elite forces are absolutely murdered by Harlequins. I was worried to start the game, but quickly found that I was able to focus on units and remove them. The Leviathan was a model I simply avoided, as I had the speed to do so, negating the futility of trying to take it out. I focused on taking objectives whenever I could, even when it prevented me from being able to engage my foe in the manner I wished.
Keeping the Death Jesters and Shadow Seers as separate units worked well. While it made them easy targets for First blood or wipe out a unit victory points it also means my opponent is wasting an entire unit's worth of firepower to wipe out a single T3 model. I really wish the ability to reroll 1's for the invul was somehow built in, because it felt so necessary as the game went on.
Grav Centurions are totally wasted against Harlequins, and since they're such a no brain force you're likely to see them on the table, and boy does it feel good running them down.
I finally lost my solitaire in a game, but my opponent came at him with a drop pod filled with 10 marines and rapid fired at point blank range. It was mostly a moral victory for my opponent as the marines did not survive to see their next turn.
Overall the Harlequins have a number of weaknesses, primarily flyers and massed small arms fire, but in the right situation they're clearly a dangerous tool.