Yea, I have been asking that question as well.
Is the IS2 a Panther (weak on sides, anti-tank only)
or is it more of a rounded up unit (Pershing/Tiger).
Solid all around, as good with AP as HE (as above).
I suggested slow reload times (historical) as a deterrant.
Making it be more of an assault tank would give reasons
to use T34's.