The Vel'Koz changes are a sizable nerf and make him feel horrible
Vel'Koz midgame is a shadow of its former self. The inability to proc his passive with his ultimate makes him feel so much weaker, in large part because he is.
"But true damage on ult! But AP scaling on passive!"
No. I tried to defend these changes with math. I put a lot of graphs on these forums detailing how he was trading squishy-bursting power for late game tank-busting, but it was ok because he'll always be able to destroy squishies. Bull****.
At level 8, Vel'Koz used to be terrifying. You could reliably blow your opposing midlaner's flash if your ult was up and they took a bad trade, because you could proc your passive three times. With one rank in your ultimate, you now need more than 120 AP to deal the same damage you used to against a target with 40 mr. Now that more of your ratio has moved to true damage, mpen is worse on you, so it's best to just rush that AP and enjoy being much weaker until you get it.
Riot, you managed to take a champion that people liked to play as and against, that everyone thought was fair, and screw with his power curve in a way that feels really, really bad.