From a personal standpoint: I like stuff simple. Between 6 AR style guns I have 2 stock types and 2 handguard systems. One grip. One sling style. If I had 10 more guns they would probably follow the formula. I try to avoid proprietary stuff. People say "more" reliable, but I have no issue with reliability on my standard guns, so "more" reliable than totally reliable seems like a sales pitch. From a functional standpoint: gas entering the carrier is what begins unlocking the bolt before it slams back. Not exactly sure if the factory piston setups address this, but the add on kits don't. A little more weight. Few more moving parts. Few more potential failure points. I honestly don't think these things are a huge deal, but worth pointing out. MY reason for avoiding them is pure preference. I don't think it's any better, Just different. KISS. don't complicate stuff with complicated stuff. If it ain't broke. Etc.