Road bikes are built to face different challenges than MTBs. Does this lead to difference in the crank strenght versus weight optimization? Will road cranks tackle to the occasional bump onto a stone on the trail?
In general, road cranks are lighter because they use less material than mtb cranks. Mtb cranks use more material so they can handle more abuse. Compatibility aside, I would be cautious in using road cranks for much more than cyclocross duty. You may never have a problem, but do you really want to find out the hard way?