They can be more than one thing (hey, a thing from the Ms Marvel show).
Wanda and Pietro. Their histories are so intertwined with both, its really hard to classify one in only one category, and its okay for others to be the same.
Beast, hell yeah he's an Avengers character. He and Wonder Man were a classic duo back in the day.
Rogue has been in enough iterations of the Uncanny Avengers that yes, she can easily be considered an Avengers character know. Hell, she's lead 2 versions of the team.
Wolverine is more of a universal character than any one, because he is in every goddamn book. But, Benids used him a lot as an Avenger, and been used enough that yes, he is an Avenger.
In short, the entire point of this goddamn mess with Wells and his Spider-man run was to say, yes, Kamala is a mutant now. And mutants tend to be X characters.