Yeah, easy to see and shoot but you think also that easy to penetrate? ISU has been made by completely other reason then giant turret of KV-2. It really doesn't matter. It was like Panzer III and StuG III.
KV-2 is nothing like the Mark III or StuG III
Mark III is a medium Tank
StuG II is a Assault Gun
KV-2 is some weird abomination Heavy tank fitted with a giant turret and 152MM gun
Barring a few sporatic incidents the KV-2 was awful and only kept on production with the KV-1 because early German AT guns couln't penetrate its armor at distance, and there weren't any other large self propelled guns at the time until the SU-122/152.