Heimerdinger's turrets are too much for this game.
He can have 3 of them, they scale with him, they regenerate, and they continue to fire after he's dead. Building magic resist against them doesn't really even matter because of magic pen. I think Heimer alone is a pretty balanced champ for an immobile mage, but the turrets themselves are what's toxic. This has been allowed to go on for way too long.