I've seen someone do the HK416 dunk test with a VFC HK416 and it kept on chugging just like the real thing.
ICS metal bodies are very tough. However, the metal they make their RIS units out of aren't as good.
At some point you've got to ask yourself what you're actually going to do to it. Roll around in some mud, maybe drop it a couple times. Certainly if you want to run it over with a truck you're in the wrong sport. CA and G&P guns should be able to withstand most airsoft punishment.