The main target of the Godot Engine are game developers. But Godot's easy workflow and functional UI elements, makes it also a good fit for non-game applications. There are already some out there you may know, like Pixelorama, an Open Source 2D sprite editor.
My 0.02 - I've been developing a code diagramming tool in Godot. It's been really nice to work with. I think its much easier to build a decent App UI in Godot than in, say, Android or (fucking) Swing.
It's not as expressive as the combo of html/css/js, but it's also much faster to get something useful put together with standard widgets.
I've been able to put together a combination of a text editor, buttons, menus and then my own custom graph-drawing widget.