Consistency in breaking. I believe one once it was set light wouldn't consistently hold. But it was mostly inconsistent tension to break the release. Never pulled any apart to see what the actual problem was. With other releases I have seen where the sears weren't milled correctly and angled. Basically a bit has a life expectancy, when it gets wore out and you keep using it, things won't be correct. Translates to sears not making full engagement on each other, which can also cause a release to break while drawing.
These issues aren't limited to any one company, just seems 2 companies I'm seeing/hearing more issues from. Could be that they are still the most popular companies out there too, so it's possible they could have a return rate half as much as anyone else, but if they are putting 5 times the releases out there, they will have a higher number of overall problems. I don't really know who is selling the most releases right now.
Personally, the amount of money they are charging for these releases, I ain't spending several hundred arrows hoping that a release smooths itself out. It might smooth out, or it might give out halfway back. I haven't busted teeth yet, but I know 2 guys that broke their own front teeth holding onto those brass knuckles.