As noted above, that's the simplest way to sum up this mod. Our party level was 7, and the final 'boss' was CR 12. Naturally, we got slaughtered.
It's a shame, because the majority of the scenario before that was interesting and fun. Lots of social skills, lots of RP, a nice little plot twist to investigate, a moderately challenging fight... good stuff that's often lacking in PFS scenarios.
But then came the final enemy. CR 12. High DR. Attacks from surprise with multiple hard-hitting attacks and combat maneuvers, and it literally cannot miss ever (+25 to all attacks). Our archer gets five shots a round and couldn't hit him (and couldn't have hurt him if he did). And you can't even run away. It was a major letdown, as the futility of even trying was evident from Round 1. The GM said he would never run this story again, and I can't blame him. If I wanted to throw away a character on a pointless, no-win scenario, I'd play Bonekeep.
What bugs me about slaughterhouse encounters is that there's no cleverness or creativity involved; there's no way to win aside from sheer power. You don't get to figure out the bad guy's weakness, or come up with an ingenious strategy. Smart tactics don't work; roleplay is irrelevant; teamwork won't save you; even luck doesn't turn the tide. All that matters is sheer power, and the bad guys simply have more, so you die. Period. Where's the fun in that?
This story had plenty of good opportunities for roleplay, investigation, and intrigue... which unfortunately had to be cut short to allow time to penetrate the vast enemy fortress and rescue the scattered prisoners by way of exploring/searching the whole structure.
This latter part of the story was well thought out too, if a bit complex for a GM trying to juggle everything (including the timing, if an alarm is raised)... but there was just so MUCH content that we had to really rush to get through everything in a standard four-hour session. And it's annoying when you have to cut back on the RP to get all the combat in (I'd much prefer the reverse as an option if time is pressing).
Really, there's enough here to be a full-length module. Trying to cram it all into a scenario-length session was difficult, even though the story itself was excellent.