As far as I am concerned they started with wrong UI design (action bar from D:OS2) and than started to add features to make it viable. I think it's biggest failure is in tutorialising the mechanics - playing Solasta I never had an issue understanding how the class works, what consumes what resource, and what options i have. BG3 is messy in that regard.
That said, the UI revision they did midway through EA was a great improvement, and 1.0 is still better than what we have.
My prefer method is:
1) remove all (red) weapon skills. Those can be displayed by pressing weapon icon (together with relevant items, like arrows for ranged weapons)
2) If melee class - organise their skills. They tend to not have much, so hotbar is usually enough to have them neatly organised... at least on low levels.
3) if spellcaster - remove all spells consuming a resource from the hotbar. Leave cantrips and item spells only, and organise them in the way so I can see which is which, and what action they consume (main/bonus). If I want to use higher level spell, I will use drop down menu anyway
4) Go to "custom" tab, and assign shortcuts for commonly used skills - so damage cantrips etc.
5) set all reactions to "ask"
6) you can assign passives to main bar - so move non-lethal attack and class specific toggles (like push eldrich blast) to main bar when applicable.
More management that I wish I would have to do (preferable would be none, except for occasional shortcuts. Hotbar shoudl be just that - a bar to access "hot" skills, not "design proper UI" toolbox)
I am sure the game will be better with time. It is a game in which a lot can happen, and I am sure something is bound to break eventually. So far, though, it is far more polished that I expected. No issues worth mentioning. The real test will be once I leave EA area, that received less public scrutiny over the last 3 years.
If companions gain their subclasses later (Laezel, Wyll, Astarion) you choose the subclass on level up. Same with PC. For some unknown reason the game automatically picks first subclass and doesn't highlight it. Abysmally bad design, if you ask me.
If you want to change the choices the game made for you and never told you about, your friendly camp ghul will do it for you.