some characterisation suffers in the x-men movies because as talented as the people behind them are, it's not a situation like the MCU whereby the most significant characters got their own movies to flesh out their characters.
captain america, iron-man, thor, hulk, spiderman, ant-man --these guys got their own movies to flesh out & develop their personalities, so that by the time the "Avengers" movie came around, it had the freedom to just be a free-for-all of action, explosions & michael bay-isms.
and really, that's what the "Avengers" films are; PG action romp fests with little character development.
yes hawkeye & widow & scarlet witch didn't get their own films (widow got some decent screen time in winter soldier though), and it's no coincidence they're some of the least popular avengers in the MCU.
imagine trying to make an "Avengers" movie and trying to cram a whole bunch of in depth character development for each of the main characters in the one film while simultaneously trying to make it 'action' enough to entertain all the short attention span mainstreamers who bring in the big bucks.
it would be incredibly difficult and near impossible to do a great job that respects each main character and equally gives each of them their due… which is what happens with the X-Men films.
because the main characters don't have their own films to develop themselves, they need to do it ALL in EACH x-men film at the same time.
as a result, what happens next is that in order to actually get some decent character depth & development, some characters get pushed to the front while the rest take a back seat.
and this is exactly what's happened in the x-men films.
the first 3 were essentially 'wolverine' films featuring the x-men. -again as a result, it's no surprise that hugh jackman's portrayal of logan is the most popular, and many people are feeling a little saturated by him now because of just how much he's the 'face' of the old x-men timeline.
also, as great as patrick stewart & sir ian are, even their characters didn't get too much development through the OT. the fact that we still enjoyed their portrayals anyway was really a result of the acting calibre of these two shakespearian actors.
but still, magneto & xavier were fairly 1 dimensional in these films.
it wasn't until x-men first class that we start the see some depth behind them - funny, as this film was originally designed to be a 'magneto origins' film go figure.
and then we have deadpool. fantastic film. fantastic casting. great character development. of course, it was wade's story.
not too many complaints about colossus or negasonic (the movie version is nothing like the comics character btw -surprised this didn't get more reactions) because everyone were expecting them to be background characters.