I can't say much aboyt polymer cased ammo in general, but I've had a very bad time with PCA-Spectrum shit.
I've found that the ammo (at leazt the 100 rounds of it that I got) wasn't hot enough to cycle the action reliabily. Maybe one round in five would produce enough gas to get the bolt carrier far enough back to load a round. About half the time (and I'm not sure how this happened, but using the same rifle and magazine with XM193 doesn't do this), it ends up with the front of the cartridge sticking up out of the magazine with the bolt carrier smashing into the side of the case. So far, I've yet to have that ammo get the bolt carrier far enough back so that the bolt will not close with an empty magazine.
Accuracy seemed on par with XM193, but I'm not shooting from a stable bench, so I a couple MOA difference in group size isn't something I'm going to be able to blame on the rifle.