Katitof - It counters all its counters except Jagd. Jagd is the only reliable counter if a-move is the only maneuver you know. EVen with a Jagd you need spotters and it can one-shot spotters. Feel free to post a replay of you countring an ISU in a close game to show your words aren't empty.
Fixed for you.
Yes, fighting ISU is annoying, no, its not OP, I've fought quite a few as both okw and wehr, no, they are not as dangerous as you try to make them seen if you were not roflstomped in early game.
2 Pumas or panzers with infantry push and they are scrap metal. Hell, a pair of normal PaK40 can scare them away.
L2P. Seriously, its nothing more.