most report says it is the Vulcan polymer upper that fails, the lowers aren't subjected to the same stress as the upper.
Just to clarify quickly.... since I have one of the Questar ARs, the upper is not made by Vulcan, but supplied from a different manufacturer. Unfortunately Questar doesn't seem to want to divulge who's the maker of their uppers, just saying it's an US made upper chambered for the 5.56...
But what I liked about it, is that it comes pretty vanilla, and I can upgrade it as I choose... not bashing the CDX-15s (which are pretty slick, but I just don't have 1399$ to dump all at once)...
The NEA-15s are really tempting as well... and for the price difference, it's well worth it...
I'd personally stay away from the Norcs, they shoot well, but their finish is lousy... unless you plan on camo-ing-it-up, then why not??
I guess, what I'm saying it depends on how you like to play with your toys... the Questar-15 will let you slowly upgrade it to your liking, while keeping the initial cost low... the Norc will get you a gun for cheap, but don't count on after market parts fitting 100% (I hear there are some minor issues)... the NEA-15 will get you a good looking gun, and you don't even need to customize it much afterwards and same with the CDX-15s
One Shot does a great job at customizing them for you too so all the work's already done for you, but personally I like to do that kind of stuff myself.
Well, that's just my opinion anyways