I've built from probably 10 different brands of lowers and I've only have a problem with 1, which was Aero Precision. The bolt catch slot was out of spec and wouldn't work with several lower parts kits I tried, however some filing did the trick.
I understand what the guy is saying to an extent, but I've built most of my lowers from Andersen and Spikes, both of which other people I know had problems with that I have never personally witnessed. I've never really heard of a DD, Colt, BCM, LMT, etc lower having issues, so that probably means that they have higher QC standards.
