I had one P415 and it was 100% reliable. I've had a half dozen piston Puritan and Minutemen. All 100% reliable.
POF is pretty hated here, but their piston design is really simple. Some people have definitely received lemons and poor customer service from them. I have the world's worst "gun luck" but all of the POF products I've owned have been 100% functional.
FWIW I have also owned the following piston ARs: Adams Arms (factory ... not a kit), HK MR556 and MR762, Caracal CAR816A2, Colt Piston Carbine, LMT Estonian and Standard Piston, LWRC (M6A1, M6A2, M6A3, M6SL, M6IC, M6IC-E, M6IC-SP), PWS Mk116 Mod-1M. Probably forgetting a few.
This is how I would range piston AR15s....
1st Place = PWS
2nd Place = HK, Caracal, LMT, Colt
3rd Third = POF, LWRC
Last Place = Adams
PWS's long stroke system is amazing. Their QC is also exceptional.
HK & Caracal's short stroke system is a close second. HK's QC and materials are bombproof.
LMT and Colt's system are 100% reliable and functional. Colt is very accurate.
POF is lacking in materials / manufacturing and QC. I also don't like their rail system.
LWRC (and this will draw the most anger from readers) is like a faux Mustang GT500. Picture a car that LOOKS like a GT500 but has a 6 cylinder engine. LWRC's lower design is AWESOME. The rest of their execution left me very disappointed. All told I've owned about a dozen of them (various models, barrel lengths, twist fluting, factory cerakote finishes, etc). They were the LEAST reliable especially with steel case. The design "feature" where you have to remove the top rail to maintenance the gas is lame. HK's system the entire rail system comes off and is "keyed" to the upper wherein you can't loose zero. In my experience you WILL lose iron zero if you remove the rail enough times (again, required for maintenance).
Adam's makes an entry level budget gun.