The problem with adapting X-Men is twofold.
First, there's the issue of the stories themselves. Take Dark Phoenix.
Claremont would weave background storylines throughout years of main plot only to have it pay off incredibly at the end. Jean Grey's becoming Phoenix and her slow corruption into Dark Phoenix was a slow build from issue 100-138 (1976-1980). For the second time now, they're trying to tell a version of that story in a single film. The reason the story works so well is book form is it's gradual nature and the fact it's based on character building. Phoenix slowly surprising everyone and herself with her building power, the newness and unfamiliarity of that power leaving herself open to corruption, that corruption building issue after issue and, even when she's able to break free, the damage being done. They keep wanting the spectacle and built in drama and tragedy of that story but they don't want to earn it through character & story. In X3, the Phoenix was just Jean's suppressed power/split personality who breaks through to randomly kill people for no reason and stand next to Magneto doing nothing for a huge chunk of film. In this film, we're going to take Sansa Grey, whose screen time in Apocalypse was a lil Scott flirting, a cut scene mall trip, Wolvey cameo & "oh wait, let me do bird stuff for no reason" at the end. They haven't built enough of a character to make any corruption mean anything in this new film.
Now I know they're making this film flash-forward to the 90s, so I'm sure there'll be a bunch of exposition about how central Jean is to the team and how her and Scott's relationship has grown over the past decade to tell everyone how dramatic and heartbreaking this all is, but it's all shortcuts to get to this spectacle of Phoenix wrecking shyt and the drama of X-Men vs one of their own. X3 shows how hollow and awful that exact story treated wrongly be and I'm afraid they're doing it again. It's a story that lends itself to serialization and lends itself better to book or TV. If you want to make it filmic, this should be your culmination of a series of movies that you have built to within the writing of the previous films. Not toss a CGI bird in a lake in X2 or surround her with birdfire in a battle and suddenly it's Dark Phoenix time.
Secondly, it's the way Fox has treated characters.
To Fox's credit, they do tend to cast well overall. They mostly get good actors but the characters they pick are mainly due to what powers they want to see on screen & who serves the plot. They use Mystique & Sabertooth in X1 to be basically Mag's henchmen. Book Mystique is a complicated character who reforms & leads the Brotherhood for her own ends and eventually finagles them into getting a government check & first series of X films she's a mostly silent shape-changing spy. The second series of X films she's basically Xavier's adopted sister, then a spy assassin (most of that happening between films and the film itself is everyone trying to convince her not to be that), then an underground freedom fighter (which, again, mostly happens between films and they spend the film trying to get her to join the team). Book Sabertooth is a cold blooded killer with a complicated past & a long history with Wolverine and in the movie he's Mag's muscle. Rogue is a scared face & plot device for 3 flicks, Storm is one of the best female characters in comics history & they can't get her right And so on...
To give examples of proper adaptations, look at Deadpool & Colossus in Deadpool, Logan in Logan & Legion in Legion. None of those are to any degree "comic-accurate". All are adaptations that take license with character to tell their own, updated stories but they stay true to the essence of these character. The main X movies to date have constantly undermined the core of these characters and what makes them compelling to just throw a bunch of random people & powers around the only characters allowed to have full arcs: Chuck, Erik & Logan.
They've been making films since 2000 and have just been retreading xavier/magneto/wolverine for the most part.
