He is from the time where the 0cs support meta didnt exist. He was labeled as a support by Riot but players migrated him to Mid as a utility AP carry that was good at screwing mobility assasins with point-and-click burst, but was shitted on by long range/poke champions due to range. A Lux could spam /l at him as he tried to get close to her to bomb her. Basically Riot nerfed him by making his Q a skillshot because he was stoping LCSBIGPLAYS from Zed.
His supporting was pretty bad (Old Karma problem, shoved into bot but needs gold to buy AP and Mana, was way better at mid), and while it was improved with the rework, they completly killed Mid Zileanand his reliability on his ONLY DAMAGE SPELL for that, and its still sub-par compared to Nami. His ult is his only good point, but even then with a better support it may have been unnecesary to win trades.
Also, his latest changes made Rewind so useless and redundant, that a charges system on Time Bomb and Time Warp could better that Rewind. That frees up a slot for another spell ZILEAN NEEDS.
He still needs copious amount of gold to buy his core, since he has to spam so many spells and land them every single one to perfection to achieve the same results as his ''peers''.