They aren't immediately obvious. I actually had to ask after trying and failing.
Maybe users who hit a roadblock could make a thread in which everything that is not possible is listed? I think that could be really valuable to users who want to know what can be accomplished and what not, as well as show larian again that there is interest for these features.
Regarding the GUI: we can unpack the files with JPEXS [0] (I tried it). Maybe there is a way for us to change and compile them so the game accepts them like the people did with Skyrim. Although that would probably only allow slight modifications but not completly new GUI elements as they would probably need to call into the engine.
[0]
https://www.free-decompiler.com/flash/features/
