As a guy who ran a Plum Crazy & New Frontier Armory poly lower for years I'm in the 'pass on polymer' camp too.
Never had any issues with the Plum Crazy but I also babied it as well. Just a range toy, and I already had two ARs on aluminum lowers.
Both lowers had all polymer parts as well. The NFA FCG malfunctioned and started double tapping, when I opened it up the pivot portion of the hammer had started to deteriorate, so I replaced both FCGs with steel ones.
Later I helped a friend put his first AR together and gave him the complete NFA lower. Ran fine for a year til he sheared off the retainer pin (also polymer) for the front takedown pin. Then we found a complete PSA lower on sale and he upgraded and gave it back. Later when swapping grips half the grip screw threads fell out of the receiver. Right now I'm just using it to hold a .300blk upper upright in my safe as I'm out of shelf space.
I tried to swap the RRA aluminum lower on my 9mm carbine w/ramped bolt with one of the poly lowers (can't remember which one) when it still had the plastic FCG. Wouldn't work, hammer would not reset. If you want this poly lower for a pistol caliber/.22 build you'll need the 'blowback' hammer if the FCG is plastic.