With the exception of barrels and high end triggers, I have yet to hear a convincing argument about how some AR-15 parts are "better" than others. It always ends up in the same place. Mil-Spec vs. non Mil-Spec, with each side beating their chest as to why one is supposedly "better".
Most all major AR-15 parts, (lower and upper forgings, complete bolt carrier groups, standard trigger components, etc.), are all made by just a handful of manufacturers who supply the entire industry. Some, like Daniel Defense, claim to manufacturer everything in house. That may or may not be true. None of us have any real way of knowing, unless you actually saw this with your own eyes in an in house tour. And even if it were true, it is no guarantee their quality is better than anyone else's. Companies like Continental Machine Tool supply many, if not most, of the AR-15 world with a variety of components they sell in bulk to all the different manufacturers. The exception of course, are specialty components like Nickel Boron treated Bolt Carrier Groups, ambidextrous safeties, rail forends, (both Delta Ring and free float), etc.
Again some will argue this, or else argue they provide different levels of quality to different manufacturers. All of it is based on hearsay without a shred of actual proof. CMT is bound by contract not to reveal who they manufacture for. So no one has any way of knowing. I've seen CMT bolts in Colt, as well as Stag Arms weapons. Common sense dictates they are the same quality. Why would they be different? It costs more money to manufacture at different levels of quality, because you are complicating the process. Both physically, and with more paper work and people in order to manage the different materials, processes, and certifications.
I have AR-15's from 6 different manufacturers that run the gamut in price........... and arguably quality. And I have never broken a component in any of them in tens of thousands of rounds, in well over a decade of shooting. That said, I don't "torture test" my rifles, or abuse them. I keep them clean and well lubricated at all times. Am I "lucky"? You tell me. In any event, 100% reliability in half a dozen different brands of weapons tells me if there is in fact a difference in quality of these weapons, it is meaningless to me from an operational standpoint. With 100% reliability in all of them, is it really going to matter which one I grab if I hear "bump" in the middle of the night?
I think there is way too much concern over all of this. None of which can be proven either way. When you look at the millions of AR-15 rifles out there, along with the millions more that are sold every year. And take into account all of the aftermarket parts sold, I highly doubt there is much, if any measurable difference in overall quality in the service lifetime of said part. If there is, I haven't seen it in well over a decade of shooting AR-15 rifles. As always YMMV.