The Isu has a long ass reload time which makes swapping targets (armor to inf or vice versa) rather hard to do. Its armor is limited and can be EZ PZ blitzed by p4 or panther for counter.
The pen on the gun isn't that great either so don't expect it to really do all that much
ISU armor is 340, panther armor is 320, p4 pen is 125/115/110, thats 40% chance to pen
at close range.
And ISU got 50% deflection damage, so even if pen was bad (it fucking isnt) it always deals damage.
And it wipes sqauds at will, it's perverted to watch.
So ye, congratulations, everything you said about the ISU is wrong.