We began our patrol rifle program at my agency several years ago. We have seen just about every make of AR rifle come through. Very few have problems, but of the ones that do, the vast majority have been Olympic Arms and DPMS. DPMS has gotten better...Olympic has not. Many of the issues with them are nothing more than piss poor QC, and can be fixed on the spot; Un or poorly staked gas keys, unstaked castle nuts, broken springs, etc...Many times, however, they are just junk out of the box. Broken bolts, broken trigger components, etc.
That being said, for a hunting or hobby gun that seems a couple hundred rounds a year, they are probably just fine.