Exactly this. Even in the nerfed state, it's better to go for a pak howi. The M1 mortar is solely for those teamgame maps where the MG42 rush is OP. Which is most of them. In 1v1, USF does not need a mortar to counter the MG42 as you are solo playing across the whole map. You ain't trying to assault the beach on Rhine where one MG42 can cover both the VP and the fuel point from a perfect green cover position. You either need pathfinders to see the position and flank appropriately, or you go for a mortar.
Purely by stats, M1 mortar is not that much worse than the OST mortar. A bit lower AOE and 5 less range. Nothing extreme. But OST mortar is in a defensive faction with long range stock infantry. 42 vision pio. And a mega-arc MG42. Everything you can hope for, to dig in.
Not even to mention that apparently a 10 MP discount is sufficient enough to warrant having no special ability.
You know what would be kind of cute? What about sprint, like a timed ability that cancels when in combat? And maybe a reduction in setup time (not teardown, so it can't easily run away from units). I wonder if sprint would even be possible on a mortar. Maybe take an extra 5 range from the mortar in return. A very maneuverable but short ranged mortar seems more thematic. Maybe in an alternate timeline.