Some AR expert (forgot who) once said that 90% of the failures in an AR can be fixed by swapping out the BCG.
I don't know about that... I'd say 90% of non-magazine related
failures can be traced to the BCG, but that's me pulling numbers out of the air. It does seem that if there's function issues that are not magazine related then chances are there's some kind of issue with the bolt or carrier- gas key problems, bad bolt, etc. But magazine problems are a big issue with semis in general.
IMO, the bolt carrier group is probably the most important factor in reliability in the AR platform once magazines are accounted for. Poorly torqued/staked gas key screws, busted bolts, they will stop a gun in a real hurry.