X-Men: First Class opens strong - we're meeting Charles Xavier, Erik Lensherr, and Raven Darkholme in their much-younger days, and while there are familiar hints of the characters we know they'll become later on, they're very different (but still nicely fleshed out) characters. Charles is something of a carefree playboy, Erik is a revenge-obsessed Nazi-hunter, and Raven is ashamed of her true appearance and dependent on her kinship with Charles. We get to see new sides of familiar characters that, combined with what we know about their future, provide a rich tapestry of depth.
And then they introduce like, 6 wholly disposable bonus mutants who all mostly die or contribute a little to the final battle but it's basically impossible to remember their names or character details beyond their powers. Also the only black guy mutant, whose power is literally "his body will adapt to survive ANYTHING" is somehow the FIRST ONE TO DIE.
This has always been something of an issue with X-Men films - by their nature, they're big ensemble affairs...but the movies really only have enough time to focus in on a few key characters. That leaves everyone else looking like half-assed window dressing, and renders everything they do pretty lame and forgettable. This issue has become most obvious in the past year - when the best X-Men films were released: Deadpool and Logan. Both did away with the excess mutant ensemble casts, and as a result both films felt like the strongest versions of what X-Men was capable of.
And while X-Men: Days of Future Past mostly avoided repeating this issue by not introducing a slew of new mutants, X-Men: Apocalypse leapt right back into it by introducing a mess of new mutants to act as Apocalypse's Horsemen (Angel, Storm, and - maybe the worst offender - Psylocke) and Xavier's new students (Jean Grey, Cyclops, Nightcrawler, Jubilee, etc.). This time, Bryan Singer went with much more recognizable characters to fill in the ranks, but the problem is that he thought that would be enough, and wound up with a film that had approximately 15 main characters and no real time to develop any of them.
Basically, we need more Deadpools and Logans, and way less large X-Men films filled with rushed introductions.
The first two films in the X-Men prequel series are built around a core idea - Xavier and Magneto have their opposing ideologies, and it's being illustrated through the fight for Raven/Mystique's soul. Will she side as a good guy with Xavier, or submit to her darker impulses with Magneto? We know that in the future seen in the original X-Men films that she sides with Magneto, turns into Rebecca Romijn, and sexually awakens every teen in America. But an incredibly weird decision was made at the end of X-Men: DoFP - to make Jennifer Lawrence's Raven/Mystique a good guy.
The most interesting part of Mystique was always her skewed moral compass - she and Magneto each had very, very good reasons to suspect the worst in humanity and act accordingly. Magneto had lived through the Holocaust, and Mystique was constantly gawked at for her blue, scaly appearance. And her response? To not only not hide her appearance, but to show it off as blatantly as possible by not even covering herself with clothing. Her existence was meant as a middle finger at the bigoted society that rejected her - so to have her decide to side with the good guys feel like a loss. This iteration of Raven simply takes humanity's disgust at her true appearance in gentle good humor, and that's lame as hell.
Actually seeing full-on "Good Guy Raven" in X-Men: Apocalypse confirms what a bad idea this was - the character has nothing to really do except act as an older sister to the new recruits, and Jennifer Lawrence sleepwalks through the film, looking as bored with the character as the audience is. And she spends a good deal of the film masking her actual appearance - and not for covert mission-type stuff, but even when she's just amongst friends and allies...undercutting the best part about Mystique.
The filmmakers have handled the evolution of Charles Xavier into the bald, wheelchair bound mentor incredibly poorly - there might have been some fear that First Class wouldn't get a sequel, so they had to take care of everything in that movie, leading to the ending where Magneto is responsible for Xavier taking a bullet to the spine and paralyzing him. After that, we see Xavier in a wheelchair, running his school for the gifted, and we can mostly fill in the blanks from there.
...except they wanted to make a sequel, and having actor James McAvoy stuck in a wheelchair the whole movie sounded BORING. Also tying him down as a school principal didn't sound very fun either, so BOTH of the things the previous film ended with are done away with instantly: Xavier takes "spine pills" that make him not-paralyzed somehow (at the expense of negating his psychic abilities, for some reason) and he's let the school fall into disrepair due to his depression over a number of the forgettable bonus mutants dying between films.
X-Men: Apocalypse relegates Xavier back to a wheelchair for most of the film, except it too needed a way to get James McAvoy running around a bit more - so they have Xavier doing battle with Apocalypse on the astral plane, where everything is the same except Xavier can walk. But worse, they figured they needed Xavier to go bald, and apparently didn't want to show the character losing his hair or anything normal like that - so they had Xavier permanently lose all of his head hair when Apocalypse tried to bodyswap him. Why did this permanently destroy Xavier's ability to grow hair? I have no idea, but I'm hoping he takes "hair pills" in the next film to make his hair magically reappear.
The bulk of X-Men: First Class takes place in 1962, in the lead-up to the Cuban Missile Crisis. It's a smart move - clearly distinguishing the prequel from the modern films by setting it far in the past and tying the emergence of mutants to humanity's fear of annihilation in the Cold War. There was an implicit promise we'd get to watch the evolution of Xavier and Magneto's relationship from its inception and watch its natural progress.
Except, for some completely inexplicable reason, the filmmakers decided to make each subsequent film take place 10 years after the preceding film - and with X-Men: Dark Phoenix supposedly taking place in the early 90s, that means we're going to be seeing the main characters (Xavier, Magneto, Mystique, Beast) all age 30 years....while the actors who play them have aged about 6 years.
James McAvoy will be portraying Xavier from the ages of about 25 to the age of around 55, which is bizarre in and of itself (given they don't really seem to be aging him up using make-up AT ALL), but the context of the film series makes it even crazier - Patrick Stewart was 59 when he filmed the original X-Men film. That means James McAvoy is due to transform into Patrick Stewart VERY SOON.
X-Men: Days of Future Past gets a lot of credit for breathing new life into the X-Men franchise - and one sequence in particular got a lot of (deserved) praise: the big set-piece for Quicksilver, which saw him breaking out Magneto from his underground prison by taking out a number of guards in super-slo-mo to the tune of 'Time In A Bottle.' It was fun, different, and elegantly filmed. And it totally fucked up the movie (and the next one).
The sequence establishes Quicksilver as something near god-like - he can basically FREEZE TIME and change ANYTHING THAT NEEDS TO BE CHANGED. Which makes it more or less inexplicable why Xavier and crew would leave Quicksilver behind on their mission to stop Mystique from assassinating the scientist responsible for the Sentinel Program. Even if they arrived at the moment Mystique fired her bullet, Quicksilver would be able to save the day. So, uh, maybe BRING THAT GUY ALONG.
And his continued inclusion in the prequels continue to make the plot simply not work at all - he returns in X-Men: Apocalypse to save basically everyone from the X-Mansion exploding, and goes on to participate in the final battle against Apocalypse...but instead of being the one sent to rescue Xavier (aka the most important thing going on in the battle), they send Nightcrawler to more or less blindly teleport around as time ticks away. It makes no sense, and now that Quicksilver is a key part of the team, there's a good likelihood all future plots will not make any sense (given if they DID use Quicksilver's abilities logically, it would be nearly impossible to maintain any dramatic tension).
It's basically the same dilemma they run into with Xavier in every film - if Professor Xavier had his powers at full force in every film, there would never be any force that could threaten the world. So in each movie, Xavier is taken out of commission in some way, otherwise the plot would never be able to happen. But - so far at least - they haven't done the same with Quicksilver. Hopefully he at least sprains his ankle or something in the next movie.