They are all as easy to work on and given the holes are in the proper places it's pretty hard to screw one up. That said there are undoubtedly superior internal parts but it is debatable that in general terms one will fail any more than the other or more correctly sooner than the other. It seems the main quality issue is in the bolt and carrier assembly which is easily changed or repaired. I'd also bare in mind that the AR is critiqued more than other firearms when considering reliability and parts quality. Right now there is a thread on the 1911 being war worthy and very little is said about particular parts or standards when we all know that the 1911 is probably as susceptible to poor fitting or quality parts as any gun made.