A piston system is so much better than the gas impingement. On my own AR's, one could fire between 10 to 15 magazines before the failures to eject or failures to feed because of the dirty system. When I fabricated a piston system and changed from the gas, I can shoot as long as I have ammo without worrying about failures of any kind.
The piston system was what made the AK so reliable.
It's the main reason I like the Mini 14 over the AR. Just hate the idea of pumping hot, dirty gases into the action.
Add-on piston systems introduce new problems to the AR system, essentially addressing a non-existent problem.
Carbon build up has little effect on quality built ARs' reliability.
I've carried both gas and piston guns in combat, and find little difference between them, reliability-wise.