I think it might be an actual tactic to surround tanks and other units with Harlequins effected by the Viel, so they become immune to templates lol.
Genestealer Cults are confirmed to be hurt by servo skulls (and other things which effect infiltrate) and Interceptor and the like. This means that any Eldar, Tau and Imperial army, for 34pts, can stop GSC from doing any deployment shenanigans, while anything with interceptor can chew through any units that show up later on.
Knights have a sever reduction in the arc of fire for their weapons, opening up large blind spots, as per the FAQ. Both guns can't even shoot at a model directly in front of them as per the diagram in the FAQ.
Also, for my KDK, if my Chaos Lord has the Axe of Ruin, and gets turned into a Bloodthirster Via Blood Tithe, I get twooo Bloodthirsters! lol. That's amazing.