For Vel'koz vs. Yasuo, on paper, the matchup should be heavily in Yasuo's favor. However, as someone who plays both Yasuo and Vel'koz a fair bit, thats not really how it works out. For some strange reason, Wind Wall does not block Vel'koz' knockback, which gives him a decent amount of leverage in 1v1, and more than enough time to get a full combo off. And once laning is over, Vel'koz actually gains a huge advantage vs. Yasuo if he has a decent tank line in front of him. Literally all it takes to kill Yasuo is 1 piece of CC from a tank, smacking him with your combo and then ulting, and theres nothing Yasuo can really do about it. Obviously if Yasuo's team is ahead and can just run over the Vel'koz team or he has a Malphite to get easy ults, Vel'koz is going to have a hard time. But in general, the Yasuo vs. Vel'koz counter is only theoretical; Vel'koz wins 58% of the time according to champion.gg.
As for all those other champions, I did say that Riot took the nerf bat to every single one of them, and some of them incredibly hard. As for why pros don't play him, I blame 2 main factors. The first and most prominent in my eyes is that he's viewed more as a high damage support due to his high base damages and somewhat low AP ratios. And he's perfectly fine as a support; he can definitely do work there, especially right around lvl 6. The second reason is one that concerns very high level solo Q and competitive play; namely that he is very weak to having too much attention paid to him, both in the form of being camped in lane and being hard dove in teamfights. Another weakness is that he's particularly vulnerable to ranged AOE hard CC, usually skillshots, such as Gragas ult, Annie ult, Sej ult, Leona ult, etc., which are all very popular in pro play.
Another thing I would like to address is the idea that Vel'koz falls off damage-wise. And while this is true, this drop off is also greatly amplified by what I believe to be bad item builds. For example, nearly every pro I see builds either
or
and then no CDR anywhere else, leaving you at only 25% whenever you don't have blue buff. For a Poke/Combo Mage, this is completely unacceptable. Vel'koz scales so hard with CDR that having anything less than 40% late in the game severely gimps both your poking and teamfighting power. Another item choice that I think is a mistake is the idea that
is mandatory. For the way Vel'koz tends to position in teamfights, the fight is usually already lost if enemy champions are able to get close enough to burst him down. The exceptions to this rule are champions that have special tools to allow them to completely bypass your team to get to you.
,
,
,
,
. Against those champions, getting Zhonyas is definitely a must just to buy enough time for your team to realize whats happening and muster some peel for you. But against most team comps, Zhonyas simply isn't necessary and using it is more likely to get you killed than just trying to kite back is.
With that in mind, what do I think people ought to be building? For me personally, there are 2 builds and which one I go for depends on what runes I decide to take. The first setup runs 9 cdr per level glyphs, and the build will give me the full 40% cdr at lvl 18.
. This build gives you a very strong mix of kiting power, raw AP, and Magic pen to go along with the 40% CDR and allow you to deal absurd amounts of damage even in the end-game. The second setup uses 9 magic resist glyphs for when I'm laning against some of the more dangerous mid champions
. In that case, I build
and then either
, or
, depending on how tanky the enemy team is. The concept is the same as the first build; max cdr, high raw AP, high magic pen, and lots of kiting power. In either of the builds, you can replace Ludens with Zhonyas if you feel it is necessary, but I pretty much never build Zhonyas unless I'm absolutely forced into it by one of the threats mentioned above.
Vel'koz is one of my favorite champions, and easily my most successful one last season: 62% Win rate, 8.5/3.2/9 KDA over 165 games, and even 58 perfect games (0 deaths) with 5 in a row at one point. I feel that he's an extremely powerful champion that is held back from his true potential by poor itemization choices and the idea that he's just a support and not a true AP mid. I honestly believe that he among the biggest sleeper OP picks in the game and has been since his release, and I encourage more people to try him out for themselves without making assumptions.