With MTB forks being so ridgid and stiff, the usual point of failure is the head-tube. If you hit a log full on, the head-tube will be caved in, and the DT and TT will buckle, but the forks will be OK
Having your forks weaker than your frame is a plus, since you can replace the forks easily. Ridgid steel forks will fail by bending at the fork crown. You can unbend them using a cast-iron drain cover, just to ride back home.
The only evidence I have seen of broken forks on the trail were some dropout tabs from sus forks which were badly engineered and simply broke off.