This movie had a lot of problems. Too many new characters that we don't have any emotional connection with. Sure, this is an X-Man movie, so these characters have a long history in other formats—even in X-Men movies that are still sort of considered a part of this series—but now that these characters have been "rebooted" and are being played by different actors, as a film you have to build those characters if you expect audiences to care. The movie never does.
Another character problem is that we don't have a character at the center, grounding the narrative (Wolverine did that in all the other films accept First Class). I understand the argument that the X-Men universe is full of characters that could be used on film, but you need to have central characters people care about, and none of the characters in the film are very interesting or worth rooting for. This is more a fault of the script than the actors, as it doesn't give any of them anything interesting to do.
Putting aside a lack of interesting characters or general purpose for the film, it's also not very fun, even in a big dumb Hollywood way. It's not as bad as say, a Transformers movie, but it's not that much better either. Overall a subpar film, that doesn't have anywhere to go. Apocalypse is a sequel that's run out of juice.