I tried to learn it a little but I did not manage to understand how it works, I don't really like the whole WYSIWYG thing, it forces the developer to surrender control, and you waste a big amount of time teaching yourself how it works internally, time that could be spent being productive with a simple renderer. It also seems to support c++ as "gdnative", although it requires another compiler to link against godot's b…
So the UI and everything are written in C++.
That seems to be exactly what you are looking for.