I am an FFL /SOT and shoot a LOT of FA in M16s. Mostly Wolf ammo to boot. None of my guns, to date, has cracked a bolt. Have cracked one cam pin though.
I am anal about inspecting the bolt and cam pin every shooting day and suggest others do so at LEAST as often.
Obviously, i do not shoot in the miserable conditions that our folks in the sandbox do. Not sure if that has any effect on the bolts but it sure wears everything it gets in (and it gets in EVERYTHING) so can't be good for the bolts either.
I am no engineer but am an A&P dealing with highly stressed parts. IMHO the bolt issue is less one of design than one of quality. I would NEVER use a plated bolt due to hydrogen embrittlement. I would never use a bolt from a questionable source. The type and QUALITY of the steel and in particular the quality of machining a bolt has a HUGE effect on it's strength! Tiny tool marks and / or poor radi can lead to stress risers and cracks. Very highly stressed parts that are "on the edge" in aircraft are beautifully finished, usually polished with generous radius at areas like the base of a locking lug. There is a reason for that - and an associated cost!
I have looked at a lot of bolts at gunshows and in customer's rifles. Some of them are VERY poorly finished and likely to crack eventually, IMHO. I suggest you inspect YOUR bolt and a quality one, such as Colt and decide for yourself if your's is "good enough".
Admittedly a better design, such as the AK is less susceptable to these kinds of problems but we are pretty much stuck with what we have. Until a better design (which would be of the bolt AND barrel extension most likely) we need to deal with it by bolt QUALITY.
just my .02 worth