The fuze and frag pattern are also something that should be kept in mind. Most mortar bombs have a quite horizontal frag pattern (if they explode while descending straight down).
This pattern is extremely inefficient against targets on near-vertical surfaces in comparison to their performance against target in flat terrain.
Even proximity fuzes don't help much.

A low trajectory (even direct fire) munition and some of the very rare forward frag mortar bombs (or air burst WP-Inc) can be much more efficient in that case.