It all depends on the design of the gas system.
In my experience with SHORT systems (like 4" and such 5.56mm platforms) nothing really works well -- in the 9"-12" systems a decent gas piston design will be MUCH more reliable than a impingment design with a suppressor on. Sure the piston etc. gets tremendously fouled and is a bithc to clean - but they will work. The direct/inpingment system is dumping hot gas/carbon back into the bolt carrier/bolt and into the upper itself -- all this crap accumulates in places you dont want.

Some gas pistons systems vent gas that is VERY visible during Night Operations -- earlier versions of one removed the advantages of using a supressor due to the flash.

That said - what sort of battlefield loadout do you beleive it should be reliable past?

I used used suppressed impingment 20", 16" and 10.5" guns operationally and I'm still here...