Excessive gas flow carries with it carbon aerosol that carries back and excessively fouls the receiver, resulting in poor ejection and jamming.
The other primary cause of jams is an over-wound magazine latch spring that prevents magazines from being fully latched in. Especially on three shot burst, the mag works slightly loose and the later rounds may misfeed.
Most armorers think that the tighter the latch, the better. That's generally true except when you are in a firefight and swapping mags in a hurry, a softer latch will more likely allow the mag to bottom out and stay tight.
Bottom line is that worn and dirty weapons may work fine on the range but not in battle.
Okay, but unless I am thinking this wrong, all this takes place at the gas port, not after. Therefore, what difference does it make whether it's impingement or piston?
Not trying to get on your case. Just asking. Because I don't know.
As far as I know, the impingement system works fine, as long as you use clean-burning propellant. (That is assuming one knows that the design is tight-fitting and susceptible to dirt, and must be kept clean; the complaint; the dagger).
The problem during the Viet Nam era was that the "Whiz Kids" decided that Stoner was stupid and that it was ok to use all the old surplus powder that was high-residue upon firing, rather than the more modern clean-burning propellant that Stoner spec'd. And the intelligent dumb@$$es prevailed at that time.