The Midnight Sons/Suns both in the original comic and in the game are a team of magic themed superheroes who deal with supernatural threats. In the game, around half the characters who join the roster are actually non-magic users affiliated with The Avengers, causing many of the original Suns (especially Nico) to resent them for shutting them out of major decisions, bringing in their own ways of operating, and just generally not meshing with the team and hogging the spotlight. It's possible that the Avengers characters were mandated inclusions to make the game more marketable, and the Suns' opinions reflect this.
On the other hand, we know that the developers had planned to put Deadpool in the base game, who doesn't have ties with the Midnight Suns either, but it was Marvel who advised the devs to save him for DLC. So having heroes that weren't part of the Midnight Suns playable could have always been the plan.
In general, the Avengers characters have simpler playstyles (Captain America: taunt enemies and generate lots of Block, Iron Man: spend your redraws to make the cards more powerful, etc.) whereas the Midnight Suns characters can be harder to get the hang of (managing Ghost Rider's self-hurting and lifesteal abilities, learning how to make the most of Magik's portals, etc.) This could reflect how the Avengers are veteran superheroes who can use their raw skill to solve most problems, whereas the Suns have less experience and need clever thinking to reach full effectiveness.
Compared to ever one else's legendary cards Captain Marvel's can seem lackluster. Hers does the least damage and is the only one that does not exhaust. Fridge Brilliance kicks in when you realize all the other heroes are either holding back, or digging deep to break past their limits. Captain Marvel is never holding back and is always fighting as hard as she can. Her legendary is just one more combo card in her arsenal, where as every one else is either breaking their limits/ or spending all their energy on trying to get a one hit kill.
Magik is incredibly easy to rack up kills in the T.H.R.E.A.T room, especially once she has Limbo's Grasp to make her portals do damage. It makes sense considering that the fights take place inside Limbo, which she rules over. For her it's less training, and more stress relief.
Hulk's playstyle is all about his rage: the more he gets hit, the stronger he becomes, but this bears the risk of him taking excessive damage and getting KO'd. A character who's good at preventing this is Storm, whose Storm Shield card provides protection to all of her allies... and who keeps a strong grip on her emotions, rage included.
Why is Banner unable to transform into the Hulk? It's possible that while Banner was corrupted by Lilith the Hulk wasn't and is thus refusing to come out. Hence why Banner needs a new gamma infusion so his brainwashed self can hulk out.
Hunter manages to make incredibly powerful relationships with everyone but the Hulk, some of which with people who are historically lacking in friends like Magick, Nico Minrou, Deadpool, and Venom. Even Peter Parker has rarely gotten any deep friends who know his true identity. How devastating is losing Hunter again?
