I have a warrior character who will need to decide on a weapon specialization. Deciding between mace and axe: cranium seems like a very fun and potentially useful skill, but how often does it actually succeed?

Cranial lags about like trip, and has a small chance to knock someone out.

With axe your lag move is pincer. Pincer lags your opponent about like cranial, but it lags you longer, so your opponent could get in multiple commands (like moving in a direction) between successive pincers. Pincer does substantial damage, unlike cranial, which doesn't do much damage at all.

it's cranial, but anyway. It's pretty reliable but that also depends quite alot on your skill percentage. Once you perfect it, expect it to work about 85-90% of the time.

(By working i mean that you don't miss. It can both knock people out and stun them.)