Personally, I think Viktor's all about gaining utility from building early on to become strong during early. Once you make it out of early, you just keep getting stronger with more utility and ap.
I think it's all about building for the most part. (This is just my belief btw u don't have to believe it, but if you play Viktor like me, this should make sense to you). The people that succeeded with Viktor knew how to build him. Obviously, rush augment 1, but double dorans and sorc. shoes also allow for better early harass. There's 2 most effective starts imo, Morellonomicon (offensive) and Abyssal Scepter (defensive). Either spamming abilities or making abilities more effective while making yourself more resistant. They also need to get 2nd augment after 1st item for mid game utility and more killing potential. This is how the winners build. There are other effective builds like rushing Rylai's but the 2 above are the ones the Koreans have been doing and the one's I'm most familiar with also.
Basically, good Viktor players, like Kuro, Faker, and Coco, know how to build Viktor in the early and utilize this build so he can stand up to his laner in the early. People that don't succeed with Viktor don't try to figure out all that he's capable of and just build straight AP against someone rushing Abyssal, like Team Gravity's Keane did in that sad game vs. Team Liquid.
To summarize, again lol, I think if you build Viktor wrong vs. your opponent, he becomes much, much weaker in comparison. Hell, this is probably why people gave Viktor the cold shoulder for so long. For example, MYM brought out Viktor yesterday and built EXACTLY how Faker did the previous day. HOWEVER, they were up against a Leblanc instead. Faker rushed Morellonomicon, but I know first hand you can't do that against Leblanc- you have to go Abyssal first (or maybe Athene's). He ended up getting Abyssal second, but by that time he had already lost lane presence and the damage was done. He had been taking harass while letting Leblanc free farm so he screwed himself.