While we're at it might as well nerf Brand and Annie too since they all do what Viktor can. Fact is, immobility (in terms of a dash, blink, etc) grants champions a lot more power in their kit.
Let me address a few things, since I am apart of the Glorius Viktor Master Race, this may seem a bit biased.
Viktor is a nuke champion. Like Malhazar, Brand, and Annie, who are all immobile as well, they have the power and potential to oneshot people, because that is what they do.
He is immobile so the only way he is going to reach a carry to oneshot is if he walks near them, which is what a tanky front line is for. It's also hard to escape anyone with a slow, since it immediately dampens whatever boost his Q-Upgrade gives.
He is a late game focused champion, who in the early game, is reliant on hitting a slow moving skill shot to get his enemy down to anywhere acceptable to kill. Dodge his Lazer early game, and a lot of his damage goes out the window.
He is item reliant. If you can deny him farm, he will slowly become lack luster, since he is heavily required on getting his upgrades. Anyone who has an all in or heavy poke champ, can do a lot to him.
Otherwise he has other limitations as any other Ap champ. Is mana reliant, has no scaling MR, heavily reliant on positioning, squishy as F**K, and is challenging to use.
So, no, Viktor is not overpowered. He is in a spot where Riot likes him, and has a steady winrate.
