It also doesn't seem to stack with slayers to a certain extent. I play a legendary mace fighter necromancer a lot, and with a silvered mace and a conjurer's trinket, I do more damage hit for hit than my concussion/crushing blow do. I'm not sure exactly what' up with that, but it seems there's some kind of damage cap for the specials. I can't get crushing blow to do more than about 95, even if I'm hitting for 100+ regularly. Interestingly, it'll do the same amount of damage if I unequip either of my two slayers (have tried with a normal diamond mace as well).
I don't have trouble killing stuff even considering that-- I can always use my necromancy spells to speed things up-- but it is a bit annoying to have 30 mana go to waste when you forget.
On the concussion note, the second damage is untyped based on... uh.... if memory serves the difference between the victim's max mp and current mp, while the first blow should be a regular hit.