Old Aatrox was an AA drain-tank with a Zilean ulti as Passive. It's a pretty big recipe for a "binary" champion who can only be either oppressive or super meh. It's kind of a no-brainer that he was a pretty big balance nightmare and certainly was far from "no real problem with old aatrox".
The new Aatrox was just re-released with bloated numbers that Riot didn't want to nerf for some godly reason, and then suddenly had a change of heart and nerfed him into the ground because again, reasons.
The new Aatrox isn't as bad balance-wise as the old one when it comes to their kits' design, that's for sure, though.