I have bought all sorts of "small" parts from them ( 308 Buffers , K-Spec buffers, Downward Vent BCG's ( just ordered a 308 one )
I have been happy with all of it.
I also got one of their complete uppers a few weeks ago, the handguard was quite rigid to me ( I didn't notice anything about the finish )
I haven't tried any of the receiver sets, but I image they would be GTG. I prefer a tight upper to lower fitment, so my goto has been SOLGW.
To be honest, if KAK sucked... we would see threads about that frequently, but I can't remember reading even one KAK sucks thread.