The writing was bad. I played through the campaign four times (three normal, one ... whatever the hell hard mode was called) and I still have no idea what "the Veil" even is, why we cared about it, why we did literally anything that we did, etc.

The new Darkness element was fun but the way it was introduced made it REALLY OBVIOUS it was supposed to be in Witch Queen and just got delayed.

Game balance went out the window, to the point where people were getting one-shot mapped by Cabal rocket launchers in patrol zones.

They introduced a new raid that, while thematically fun and visually gorgeous, was un-fun to play.

Eh... the more I Think about it the more everything Chozo said covers it more eloquently.