It’s simple: The IS-2 can penetrate the front of the Tiger II (H), the ISU-152 can’t.
The fact that this game allows for countless impenetrable tanks to be spammed with the same 2 vehicle cap as any other tank, combined with the complete absence of coordination in the game, forces me to always choose the single vehicle that can penetrate a diminute region of the turret, instead of the fun assault gun that i just earned.
Giving the ISU-152 another squad would allow both to be used. This, or maybe make the top armor of the hull to be destroyable by a 152mm HE the same way that the Tiger I can be destroyed by the 105 Sherman HE shell.
152mm HE absolutely can penetrate the roof armor of Tiger 2 H and overpressure it. But you are right about ISU-152 being so restricted in usage by squad misplacement.