IMO, Paizo tended to play too much of the "arms race" game when creating new monsters or NPCs. I noticed this a lot in the Rise of the Runelords AP. By "arms race" I mean, the general consensus is that good players are using tactic X, so we'll create a monster that is immune to tactic X. Then when players switch to tactic Y, we'll create a monster that is immune to both tactic X and Y.
Eventually you've got an opponent who lasts the 8-12 rounds the designers apparently want it to against the PCs but only because they're all reduced to wailing on it with their most ineffective attacks while burning all their healing powers to stay alive long enough to finish it. Those kind of grindy combats aren't fun for anyone and I only really noticed it against opponents that were custom-made by Paizo.
By comparison, most standard Monster Manual opponents die very quickly vs. fresh PCs with good players. However, our DM was able to simply add more opponents of the same type if he wanted a tougher combat. To me, that's a much better fix to the problem.