Is there a reason you wouldn't use a game engine to create general purpose cross-platform apps? Is it cumbersome to create textual, structural interfaces (e.g. a twitter clone, google sheets clone, or an ecomm checkout flow)?
So it is absolutely possible to create complex applications with it.
Games engines are generally not used for apps, as games are optimized differently to apps. Games need to run smoothly at a high refresh rates at all times while apps can not hog all CPU power but need also be optimized to not needlessly drain the battery. This is no an issue in Godot specifically because it offers a low processor usage mode though.
So yes, Godot is a decent solution for apps. Only real issue is support for accessibility. There are screen reader plugins but if you want to do it properly, you might be better of with an app using native elements.
Apple's own UI libraries use their graphics engines too(SpriteKit, SceneKit), unfortunately supported only on Apple platforms.
For example, here is kanban PoC made by someone:
https://github.com/alfredbaudisch/Godello
But for web, that could be too much for browsers to handle. Godello loading at webbrowser has only some time watching at spinner, when you have fast Internet connection, loading about 10 MB of code, WASM etc.
But when I tried to save Godot 3D game example to HTML5, and load it to webbrowser, Chromium etc browsers can not handle that much data.
Some webbrowser users have slow connections. If there are many users, they start to complain webpages loading too slowly. With Godot everything is browserside, and generated with save button, from that it's hard to move anything to serverside.
But if you instead code frontend and backend with Javascript, it's possible to move more of frontend Javascript code to run serverside.
The tech that is often used in game engines, especially related to GPU support but also memory management and GUI structure is often 10-100x faster than what is commonly used elsewhere.
Text rendering can be just as nice as any desktop app, it is not trivial but not rocket science either, and quite a few smart guys have been doing nice work in this field.
> ...quite a few smart guys have been doing nice work in this field
can you expand on that? like creating quality of life improvements to existing tools like Godot, or something entirely new but in the spirit of game engines?