Is the roll for a combat maneuver really supposed to be KAC+8? That seems herculean by even soldier and solarion standards. For a level 1 soldier (BAB +1) with max STR at generation (18, +4 Mod) this puts the ability to hit a basic 1/3CR CASTER array (Alien Archive) that has an unmodified E/K AC (KAC 10) to 18, meaning the soldier has to roll a 13 just to pass, and that only gets more out of hand as the game goes. The basic COMBAT creatures/thugs you might encounter very early on usually have KACs at 12 or more, meaning you're looking at maybe 20+ needed at LEVEL 1, not to mention that you'll probably NEVER hit the 5 over bonus.
Keep in mind, those numbers are also for peak stats too.
It seems WAAYY too punishing for something like that, and almost forces you to feat into specifically performing CMs.
Yeah, it does seem really tough to pull off. I've yet to find a time where combat maneuvers really feel worth it. Even when it's only an effective +4 with a feat.
The basic penalties are rough for non-ininitiates but specialized maneuver builds have merit. Say the soldier in your example has Improved Disarm and is wielding a taclash. If he tries to disarm the caster of a spell gem he'll only need to roll a 7 or better, that's pretty decent odds.
Granted, I find that a lot of maneuvers are situational - disarm is very effective if you're primarily fighting humanoids, but rubbish against monster enemies. Trip is useful if you have an all-melee party, but knocking foes prone is basically sabotage for your allies with guns. Dirty Trick is interesting but it's a shame it can be fixed with a single move action - trading my standard action (with at best a ~50% chance of failure since there's no taclash equivalent for DT) for his move action seems like a bad trade most of the time - the best use I can think of is to delay until immediately after someone's turn and then try to blind them.
Then again, a taclash is dirt cheap and never needs to be upgraded since you only use it for the maneuver bonus. It's not hugely negative that Disarm is situational when it only costs you a single feat in a system where most martial characters have feats to spare. YMMV may vary I guess.
Well, it's really easy for the NPCs to land those - and very entertaining for the GM.