jesus Christ it fires a damn 152mm round!!! The KT and JGGT rape tanks too!!! It also has massive armor!!! much more than the ISU!!!
If you wanna talk about realism ISU had terible accuracy and was not that good against tanks. KT will have no problems with ISU. But gameplay > realism
Problem with ISU is it is ultimate counter to everything except planes. I think current call-ins system need some rework. Will you build Panther or call-in Tiger etc... ISU should by more assault gun then tank hunter. If your opponent blobs infantry get ISU. It should have more specific role as anti blob weapon and not all around performer.