There are many aspects to a well-built AR15 that slip by most hobbyists or home project builders, which I openly admit to being. I haven't purchased an off-the-shelf AR15 since 1997, but I have learned an immense amount of information about them since then that I was totally clueless about, and I continue to learn more every opportunity I get.
My approach to parts sourcing is an ongoing QC process. Receivers, barrels, extensions, uppers, bolts, bolt carriers, carrier keys, lower parts, extension tubes, recoil springs, ejector springs, gas rings, gas blocks, extractor springs, extractors, charge handles.... None of these parts are standard in the industry in the way many think they are. Sure, you can smash them all together and end up with a firearm that looks like an AR15, but materials quality, dimensional uniformity, coatings, treatments, and assembly methods are not all the same.
As I have learned more about these aspects of the AR15, I have evolved in how I choose and assemble parts for my blasters. At the moment, there are very few companies I can trust to kick out a well-built, standardized, quality product, but if I was only equipped with the level of information I had after studying and using the AR15 on a daily basis for just 9 years, I would end up repeating the same mistakes that I see almost every AR15 "manufacturer" making.
I didn't know about steel grades back then, but had read a little bit about it and assumed everyone used the same 4150CMV steel with chrome lining for a claimed "Mil-spec" gun. I didn't know anyone was making receivers out of aluminum in 6000-series, without a Type III hard coat anodizing, that penetrates deeply into the surface for a harder, stronger receiver. I didn't know about dry film lube inside the upper.
I didn't know that practically every AR15 copy had crappy trigger and lower receiver components, with corners cut on every piece, down to the zinc-plated detents, which are supposed to be cadmium-plated.
I didn't know the carrier key was supposed to be a softer steel, with hard chroming inside the key. I didn't know the AR15 bolt was never meant for 5.56 NATO, and was engineered around a much weaker cartridge, the .222 Remington. I didn't know they had realized that 8620 steel doesn't really cut it with 5.56 pressures, so Carpenter 158 was adopted, with a strict QC process for the metallurgy in order to deal with 5.56 pressures.
I didn't know that my military guns had MPI'd lower parts, with inspection and proof markings on them after passing these processes. I didn't know the pistol grip was different on my work gun, vs. my Bushmasters at home. I didn't know that specific steel grades were used for the wire in all the springs on a true Mil-spec gun, while after-market guns used whatever they could get for the cheapest.
Name a part, and I could go on about it and how different it is from what I look for when I source parts for my projects.