"I just completed an Urban Rifle Course in UT. Students brought the usual assortment of AR-15s and Kalashnikovs. We also had one RA/XCR (mine) and one DSA/FAL. All ran fine for the duration, except for one of the AR-15s, which was a gas-piston model. All the other ARs were conventional Stoner System (pressurized receiver) models, and all experienced no more than the usual number of hiccups. However, the one gas-piston ARs displayed many unscheduled interruptions, mostly failures to eject. We all made a mental note that this is not a rifle any of us would want! Unhappily, this experience has been typical at our UR Courses. As a rule, gas-piston ARs do not hold up nearly as well as conventional ARs. To add insult to injury, gas-piston ARs are a good deal more expensive than are standard models!
It strikes me that, in their enthusiasm to maintain the classic AR-15 profile, designers attempting to equip this rifle with a gas-piston have produced both a piston and op-rod that are tiny when compared with those found on the XCR, SIG/556, and other military rifles in the same 223 caliber. Apparently, a gas-piston system that small is below the reliability threshold, because we can't seem to keep them running satisfactorily.
The original Stoner System (pressurized receiver) has had a disappointing tenure. Compared with gas-piston systems, like the Kalashnikov, it has been excessively maintenance-dependant, because so much garbage ends up in the receiver. Gas-piston rifles don't get nearly as dirty, nearly as fast. However, in my opinion, gas-pistons and op-rods must be substantial, even on rifles chambered for 223. Tiny parts and tiny systems do not reliable rifles make! /John"