This^^
The TDP doesn't just specify dimensions and tolerances, it also specifies manufacturing processes, testing protocols and materials used.
While the TDP is certainly not the end all be all of the AR world, companies that meet or exceed the TDP certainly produce more reliable product/parts than companies that skimp or take shortcuts in my experience.
For a long time Colt was all there was in the quality AR market. Now with Noveske, Knights, LMT, BCM, Daniel Defense and a few others there are many more quality options to choose from that meet or surpass the TDP.
DD makes a great product. A little expensive and I have seen a few of their by carriers shit the bed, but on the whole, they are quality.
LWRC also makes a good product from what I have seen. I have taken a few classes with shooters running them and have met their head Armorer. Quality product, no corners cut - but still
have piston issues, I have seen a few disintegrate on the line.
LWRC and HK are the only companies I would buy from if I was in the market for a piston gun.... Which I'm not - just so we are clear
DPMS I avoid at all costs. Allegedly the .308 rifles are GTG but I have no experience with them.