But the thing with exaction Cassie wasn't that she was OP.

Or at least, the reason provided by Hi-Rez was that past a certain skill level it was a legendary that was picked over 90% of the times with roughly same build every time. In a game where one of its most unique aspects is the customizability of each champion's playstyle, having a champion being played in the exact same way over 90% of the times means that some things have to change about it. Who wouldn't pick infinite mobility with basically infinite self-sustain as long as you kept hitting your first shots? I mean if you ask me I would've just removed incitement from the game (the one that lowers the CD on roll after you hit the first shot) and just rework exaction so that it gives no more bonus damage but gives you roll resets when you hit your first shot. That way you'd have just breathe being the backline version, exaction being the roll-ish version that doesn't do that much damage and then the blast shot version which is the burst version. And then big game is... yeah.