The problem with this is that not everyone will like a certain style of UI, so it is impossible for a developer to please everyone with the UI. So the best they can do is give it modding capability so people can set it up as the like. What you like, is what someone else might hate. Which is why this is one of those concerns that makes no sense.
SWTOR didn't launch with UI customization. Also, SWTOR had optimization issues especially in bigger groups, it was one of the biggest complaints on the beta forums.And yeah, game breaking bugs are critical to fix. And for a side note. SWTOR played smoothly for me even with all the settings to max. But I do know it was a concern for some. One of my big complaints about it was the feeling you were on a rail in the game. Did not have the freedom to explore like you can in most other MMO's. But that was only one of many. The UI however was never a concern for me. Even today, I love how you can customize it in SWTOR.