I would like to retract my comments on this. Lol. In the Underdark, the game was glitching hard and causing even those who have darkvision to not be able to see anything. In other words, the lighting in the Underdark was glitching and plunging everything into total darkness. Oh my gosh! Be careful what you wish for. Very frustrating. Couldn't see a gosh darn thing. It was so frustrating just trying to find the enemies on the map to target them. I'd usually have to wait for the game to stop glitching and make it brighter again.
That said, I think for me it's the light sources that bother me. Sunlight streaming down even into the depths of the Underdark. I do really like the ambiances for many areas in the Underdark. The more I was exploring in Patch 6, the more I was really liking the feel of it. Still, there were patches where beaming sunlight was pouring in. Made no sense.
There can be lighting so the players can see without having some blinding sunlight source of light. And that, ultimately, is what I'm looking for. Places where it makes sense, yeah. No problem. The owlbear cave in the area where the Selune statue is, that makes sense to have sunlight streaming down because she's a goddess of light. The Dank Crypt, I can kind of see that because it is so freaking old. The Underdark? No. You shouldn't have sunlight streaming into the Underdark. Same with the Whispering Depths and other places where the monsters are dwelling there because they hate the sun and are avoiding it.