FTFY.
The mortar actually was game breaking at worst and imbalanced at best. T0 AT gun is not gamebreaking, but it really undermines the importance of teching making it awkward.
Alternately, it could've feasibly stayed exactly the same, but been put into LT tier.
It was game breaking because it was performing much better than an ost mortar... imagine if OKW had to go a certain tier for an AT gun like USF does, mechanized for example ... it would be much more awkward for them
Placing it in LT tier is even worse because it doesnt give USF a way to counter garrisons/mgs and okw still gets free nades/weapons unlock with tiers. and even with the mortar usf is still lacking with a versatile amount of units and abilities compared to the vanilla factions, lacking basic tools...
If it was just a T0 ost equal mortar it would be completely fine and not at all game breaking.