A lot of those "Frankenguns" have the same stuff with different names on the parts. The PSA kits I was thinking of have pre-built uppers which have been test-fired at the factory. How someone could mess up building a lower... You'd have to try pretty hard. Personally, I'd do the lower build myself with a PSA rifle kit and get a CL/CHF barrel by FN way before I'd get a non-CL barrel in one of the factory entry-level ARs. YMMV.
Oh trust me, people can screw up building a lower. Especially the guys who are going to save a quarter by buying a parts kit with no name on the bag because "they all come from the same place anyway". I agree with what I think you are saying though, that a well put together and tuned upper is the important part of the equation.