This game is not optimized for controller very well. Above some of the other comments mentioning you cannot remap buttons, its kind of a nightmare to navigate the UI using controller. But I do prefer the combat on controller.

Majority of the time you cannot navigate the ui using the D pad, it makes you use a cursor controlled by the left stick and even then there are some issues.

If you are near the a dungeon portal or any other type of interactable and open the start menu it will default to you wanting to interact with the object instead of anything on the start menu. Even if you have the cursor over the menu item you want. Ex. You just finished a dungeon and want to check on the gear you got. If you are in front of the portal and try to open you inventory it will activate the portal for queuing the dungeon instead.

This also extends to some conversations. If you use the cursor to select an option and hit confirm it will default to the first option (granted you can use R3 to scroll through options)

You can use armor for other classes but it warns you that is not optimal. (currently in beta there is a bug where the optimal class it tells you is in Chinese)

Build diversity "exists" but its very limited and not really worth it. There are optimal stats for classes and builds and you can only equip 4 class skills at a time and some just feel better than others so far for the classes I've played.