All gas operated guns have to poop somewhere.
Direct Impingement poops where it eats, but it's in a location that at least is convenient to clean up after. All else being equal, the DI system probably will fail to function due to accumulation of poop after fewer rounds fired than an OpRod (gas piston) system.
Typically you have to remove the OR gun's handguards to clean up its poop, which is more involved than breaking down the DI gun to access its breech. But that's an action that typically is needed far less frequently than cleaning the poop from a DI gun. I haven't heard a credible real-world field report on them yet but there are a couple of piston ARs and piston conversions for ARs that advertise they are "self-cleaning."
The bolt in a DI gun gets much hotter in operation than the bolt in an OR gun, too hot to handle, which can be a deal-breaker if for some reason you might need to do emergency maintenance on the bolt (probably not very likely).
The OR system necessarily puts reciprocating mass outside the centerline of the bore, which potentially can negatively impact accuracy.
The OR system has additional parts, additional weight, additional complexity, additional manufacturing costs and additional potential single points of failure.
The OR system also will spray hot poop over whatever happens to be near the gas block when it releases the gas. This might be you hands or gloves or whatever you have hanging on your accessory rails. In the real world, the OR gun's hot poop has been known to overheat handguards.
OR platforms are fussier than DI about gas port tuning if you try to run them suppressed.
If you've heard the nightmare stories and have qualms about the DI platform's reliability, check out this video. It's an M-4, being fired full auto as fast as the operator can change mags:
[video:yahoo]
http://www.youtube.com/watch?v=Kzfm4pYhIyY[/video]
At around 400 shots fired, the handguard begins to catch fire. At about 420, one round cooks off. Except for that one cook-off (and the bit about catching fire), the carbine functions normally until its gas tube melts after firing 810 rounds (27x30) in just over four minutes. That's about 190 RPM in a platform with a recommended sustained rate of fire of 12-15 RPM.
I think Stoner pretty well knew what he was doing, when he went to DI. You obviously avoid the piston rattling next to the barrel.
Stoner wasn't married to DI. He built the Armalite AR-18 as a piston-operated advancement on the AR-16.
EDIT:
If my media tag for the video isn't working, it's
here.
