>How has your experience been with Godot as a way to build apps so far?
Building the actual GUI is a pain. Godot isn't really optimized for that, so complex grid-based layouts and text flow don't work the way you would expect. Themes and styling, and loading fonts especially, is kind of awful. I wound up writing my own ad-hoc text based data format rather than using Godot's own system. The most frustrating thing beyond that was dealing with HN's API, though. Godot doesn't support Markdown or HTML, though, it supports BBCode of all things, and HN's API emits raw HTML, so I had to write a messy BBCode to HTML converter. Upside is links work.
I started using C# but abandoned it for GDScript because I found myself working against the engine by importing things through NuGet like the HTTP libraries, and the end result was just too much unnecessary complexity and size. A current release build is about 39Mb with GDScript, but IIRC about twice that with C#. If I were working on a serious app, though, I might consider using C# anyway, just using Godot as a thin presentation layer. But GDScript works fine for an app that mostly just pulls JSON from an API and writes it into textboxes.
It took me a bit to really learn how the expected signals-based architecture was supposed to work, and then realize that what I really wanted to use was callbacks, because the relationship between API and elements was 1:1 rather than 1:many. So anyone building apps is going to need to know when the typical game architecture advice works against them.
Other than that, it's a blast. I've seen a few applications show up on /r/Godot so people are using it that way.
>Are there any declarative frameworks like React?
.....not really. It's still a game engine that just a few people are hacking apps on, but I don't doubt someone is going to write some plugins along that line in the future, if it becomes more popular as an application framework.