Very few of us will ever see the day where we rise out of a swamp to fire the rifle at the enemy. Even so, there is a good possibility the water logged gun will blow up when fired. If the piston blows up, the failure point will probably be at the gas block. If the DI blows up, it will probably be at the upper receiver. If I were put in that position, the further away from my face, the better. Barring me getting deployed to somewhere with that scenario, which is very unlikely, I would rather have the weapon with the least amount of moving parts and the least weight, which is the DI. From what I understand, the spec ops guys are glad to have the HK 416, but the rank and file military would just as soon not have the extra weight. We have enough crap to lug around. Every microgram feels like a ton when you start having to ruck your gear around. Like Jon said, a properly built DI with quality parts will do the job as well as a properly built piston. I have both, and they work well and I spend an equal amount of time cleaning either.
"Democracy is two wolves and one sheep voting on what to have for dinner.
Liberty is the well-armed lamb contesting the vote." - Benjamin Franklin