ISU-122 is ISU-152 with 122mm gun simular to IS-2 gun.
They put 122mm guns because they didnt have enough 152mm guns for all produced ISUs. 122mm gun was efficient after all
I think it isnt that hard to put 122mm IS-2 gun in ISU in-game. ISU-122 can be put in commanders with ISU-152. So player can choose between ISU-152(less penetration, more splash damage and accuracity against infantry units) and ISU-122(more penetration, less splash damage and accuracity against infantry units).
SU-122 is a SU-85 with 122mm howitzer gun. It was produced and used as planned.
SU-152 is 152mm gun on KV chassis, while ISU-152 has same gun on IS-2 chassis.
I'd like to see more armor units for USF too