The accuracy against moving targets is incredibly frustrating. Mortars are supposed to be good for beating down a static opponent. They don't stop a charge before a bullet has been fired.
... And the vehicle damage is pretty retarded.
As said before shoots don't follow units. If you are moving in mass something may get hit due to the quantity of targets in particular area but not because shell was locked on a unit.
Being mobile is the best defense against any mortar in the game.
As far as Soviets go I am really sorry OKW has a heavy counter to your Maxim spam. Not that I had any problems with it even as Wermaht but it's good to see such a cheesy strat being countered as much as it shouldn't be so effective in the first place.
The other thing is that I wouldn't go weapon teams heavy against OKW as you will only feed fuel.