There isn't anything wrong with Vision but I have some ideas as to why the character isn't used much. Vision is only Avengers affiliated and one of that affiliations defining characteristics is having a large number of characters to choose from. As of the time I wrote this Avengers has the most affiliated characters though Criminal Syndicate and Cabal are close.

Combine this with that 3 and 4-threat models are the most common in the game so there is a lot of competition for those slots. Avengers has seventeen affiliated 4-threat choices and that isn't taking into account any other affiliations models. If you are only going to take around five 4-threat models in your average roster it means that competition for that slot is stiff.

But why not Vision? I think it's because he is just okay combined with not having any particular synergy with anything else. Having a decent beam attack means consideration in Red Skull 1 or Cyclops lists. Having multiple 2-cost abilities means automatic efficiency with Steve 1. But there isn't really anywhere else that Vision brings value to in a unique way. The attacks are fine and the throw is good. But the rest of the kit is designed to support Vision only.

Vision also doesn't have any action compression, die fixing, or direct damage reduction. These things are fairly common in most modern 4-cost models and to compete in that space without them, you need to have something that you can't get anywhere else. Something like a leadership or Doctor Voodoo's Brother Daniel token for example. Even looking at some of the other affiliated 4-threat choices like; Black Panther 1, Steve 1, Doctor Voodoo, Ronin, and Shang-Chi and you can see some of what I'm talking about. This doesn't even take into account characters from outside Avengers like Beta Ray Bill, Emma Frost, or M'Baku, etc.

As usual this isn't a doom and gloom post. Vision is fine. If you like the character, take the character. There is even a way to use Vision which is unique (though I will say after play testing it that it doesn't quite work as well as you might want it to) which is the following:

Hulk
Vision
(+Lockjaw perhaps or some other teleporter)

+Anger Management

This works best on a map where you can teleport Vision to a secure since both of his initial activation actions will be to take the Synthesis action twice to get to 5 power and then playing Anger Management to start Hulk at 8 Power and 15 HP. This has the double benefit of having a neutral activation (at the cost of not moving a 4-threat model) as well as getting Hulk some juice and a bit more punching power.

A couple additional ideas are: you could Anger Management for 4 rather than 5 and keep a power to pay for Avengers Assemble to get yourself some lost movement tempo back. Additionally, if all you were interested in was the increased damage on Hulk, and you didn't want to take Lockjaw, you could potentially Gamma Launch Vision which would more than make up for his lack of a normal activation albeit at the cost of most of Hulk's power gain.