I’ve been playing on Siptah for months, and although the lighting is sometimes less than pleasant (the Isle of Dusk is understandably perpetually dark, and in the Maelstrom the ambience is quite oppressive, which I think is as it should be), it’s far from “unplayable”.
Darkness is dark, but “pitch black” in only a handful of locations. The solution to those places is - carry a torch or other light source.
May I suggest that the issue may be with your game or monitor settings?