UI definitely needs some changes/tweaks. As neat as it is to have UI elements to be part of the world, I don't think it works very well.
Biggest issue is the tool belt. Highlights for items can become impossible to see depending on lighting conditions, and you aren't told what the highlighted item is (syringes can look identical, again due to lighting conditions).
There's also the issue where your camera angle matters for seeing what is highlighted. Items on the left side of the belt tend to be harder to see than the right side. I have to move my camera around to the sides to get a better view of what I'm trying to select. I would rather have a good old reliable radial menu.
Player status bars (the hp, stam, stress, etc bars) that float in the world are also something I would rather have as the more traditional static location on the screen for better readability.
Even with the explanation guides, the sex scene UI is still a bit confusing/hard to read. Mainly the mini game part splitting the other bars in half.