3D GUIs are a lot like graphical programming. Just a giant black hole that has been sucking up research and investment money for 30 years without much to show.
The game industry has been using 3D GUIs and graphical programming successfully for years. Graphical programming is also popular in some other fields (MaxMSP for audio visual, labview).
Add TouchDesigner to that list, which is basically MaxMSP for demoscene-style visual effects. It gets used for fancy things like this: https://www.youtube.com/watch?v=0zMI-qbmIPk
Which reinforces the point, 3D GUIs are a lot of fun but not very productive. In my workflow at my day job I want fewer GUI interactions and more production. AutoHotKey and 5 tonne overhead cranes help a lot, 3D GUIs not so much.
I won't discount the possibility that there will be some future problem that will be solved by a 3D GUI, perhaps astrogation.
You do get some neat demos, but not a lot else.