An adjustable gas system means there is a design problem. Why have a gas system that needs to be adjusted. Better to make it strong enough to work, and buffer it as needed when all the power is not necessary.
I purchased and mounted three different piston kits on AR 15 rifles: Adams Arms, Osprey Defense, and CMMG. All three performed as advertised, but of the three the Osprey was far and away the most maintenance free in a few thousand rounds. The Adams Arms is the kit I prefer, since I use that rifle to run a CMMG stainless .22 conversion kit and the piston rod comes out easily and the gas can be turned off completely when shooting dirty .22lr ammo, so no debris gets up into the gas cycle. The Osprey is darn near self cleaning, with the simplest and most efficient system of the three, if all you intend to shoot is 556 or 223. I would like to mount a 5.45 x 39 with the Adams Arms and see how it runs, but not writing a review of that so can't afford the equipment.