Always the same bunch of X-men
They have such a huge pool of mutant characters to choose from but it's always the same underwhelming team - Beast, Cyclops, Storm, X, Magneto and Mystique. Not all of these are uninteresting, but we've seen them in so many movies now and know what they can / are going to do.
Quicksilver is a great character and steals the show, but didn't really have much of a role in this. Why not show some new mutants with some cool powers like they did with him in DoFP?
Apocalypse already did the phoenix thing at the end in a way, so maybe this movie was too soon to be successful at the BO. They could've made another 2-3 movies with new characters and villains like Sebastian Shaw, with only a few of the core team still present for familiarity's sake.
Some of the scenes were great, but Sophie Turner doesn't have the charisma or acting chops to carry a movie like this, esp. when it's poorly written to begin with. Both this and Apocalypse are the same for me - some cool scenes but overall, not an amazing experience like First Class and DoFP.