The failures I have seen most are gas key screws loosing and in a few cases, breaking. If the key gets loose for any reason the gun will short cycle and fail to function. Easy fix, but some men don't know what to look for.
Next would be oblong cam pin holes in bolts that have been used in full auto a LOT, which heats them and lets them cool over and over. I have never seen one fail personally, but I had a friend in the armory that had 6 of them in a box that had cracked through the cam pin holes. These were all from un-maintained m16s that should have had the bolts and barrels replaced long before this kind of thing happens, but in some countries such maintenance is just never done. I have seen the bores on many of those rifles and the rifling is shot out in front of the throats so you can make a guess how many rounds they had fired.
I have seen 4 broken extractors. All broken at the hooks.
One broken ejector. This one broke in half right where one shoulder of the pin cut-out is.
One broken firing pin tip.
I have a friend that has an Colt AR (Civilian gun) that had one lug break off, but he kept shooting it for a few years. It didn't seem to do any harm at all. I wanted to replace it and he said he's let me sometime, but for about 2 years he just kept using it as it was.
He finally got a new bolt and put it in.
The problems you listed can be worse on cheap parts. So it sounds like the answer to his question may be "yes"..... Depending on what he considers expensive. Colt / BCM BCGs aren't cheap.
