I have to agree with the OP. In adventure mode combat I regularly "shatter" bones, only for them to remain functional. ie An ettin with a shattered lower leg bone has no problem standing and fighting. Strangely enough, destroying tendons has more effect in dropping enemies than bones do!
Even the term "shattered" is misleading. I would imagine that a shattered bone would be totally pulverised, in several pieces, incapable of any action. The way the game interprets it, it sounds more like a hairline fracture to me. And oddly enough, a shattered bone can be hit and shattered again, with no indication that it worse off than before. This is mostly evident when using a hammer on a creature: since the hammer can really only kill by smashing in the head in this version, it can be comical to shatter the skull three or four times before it randomly sends a piece of skull through the brain, killing off the enemy.
As it is, creatures routinely shrug off attacks that should kill them. I have, for example, torn a naked mole dog's heart with the aid of a spear. It just keeps attacking, instead of hosing me down with every drop of blood in its chest and keeling over. Lungs will have some effect, eventually, but not in any timely fashion. Hacking muscle, fat, skin is a waste of time. Blood loss doesn't seem to affect the enemy much. A thrust to the neck that tears out the throat only resulted in oozing blood. Creatures are almost constantly in extreme pain, but it seems irrelevant since they never pass out from it. Hitting the guts will make an enemy vomit butdoes not seem to hasten their demise overly. If a fight goes on long enough, a creature may pass out due to exhaustion. Damaging the brain is remarkably effective: even a bruise to the brain is sufficient to kill a dragon.
Overall, I find swords the best weapon to use thus far. While I've yet to set up a good decapitation, they are surprisingly effective at chopping off limbs (which isn't as devastating as you'd assume) and the entire lower body (which is instant death.).
The damage done to adventurers is sketchy, too. So far, after fighting werewolves, dragons, ettins, wolves, naked mole dogs and others, the only creature that could inflict any damage at all was a bronze colossus, which, may I say, was very effective at doing so. Every other creature could not inflict even a scratch to my adventurers.
I hope that my observations can be of help to those intending to evaluate or adjust the system.