In BfA they removed the ability to swap out gear while in a mythic+ dungeon. This made it impossible for classes to have a setup more favourable for aoe and packs, and another for bosses for example.

This limits several classes that have a distinct weakness but another strength. I think it's good that classes have weaknesses, but limiting the gear removes the ability (and fun) of customisation and adapting your character to the situation.

For my warrior I would love to have an aoe Seismic Wave azerite set, and then a single target set for bosses. But now I have to choose between weaker cleaver and stronger bosses, or the other way around. (Bad example due to bladestorm honestly, but there are others)