The weaknesses and big weaknesses you didn't mention is that it is awfully inaccurate and fires very slowly. If you keep relocating your forces, you will be fine.
This is not true. Its accuracy is the same as that of other mortars, and the rate of fire corresponds to leigas. Any mortar will miss if you move the army.
1) "you didn't mention is that it is awfully inaccurate" is the "weakness" of all mortars, and not only in the game. Although it's funny that you probably think that German mortars are more accurate and that they have homing mines.
2) "and fires very slowly" - Reload duration
8.90. It's not "very slow" given the huge splash. Then add the OKW headquarters "Brace Structure" like all Field Buildings UKF. It's not very fair when all the allies have an overwhelming superiority in artillery, but the British were whining and got the Brace Structure. Leig destroys a regular bofor much longer than a 120 mortar destroys OKW headquarters.
Either the German artillery buff or the allied artillery nerf.