For there being a surprising amount of things that can go wrong in an AR upper, I rarely hear anything about general problems from specific manufacturers (aside from PSA). But then again, I hear people constantly shitting on whatever-brand lowers they have while playing mixmaster with bolt carriers/gas keys, buffers, springs, etc.
It makes me wonder how many lemon uppers are out there, with everything attached to them essentially being a factory nigger-rig just to make the rifles work (that often require "tuning" by the end user).