I think egui is complete enough to use in real apps. Layout can be a bit weird, but I haven't found anything that's impossible.

I might even prefer it for cross-platform apps since startup time is so slow for GTK on Windows.

The challenge with egui is the theme. It looks ok, but isn't good enough for a commercial business app for example. Also, I believe it lacks a webview, so if you are in a pinch the only thing you can do is create your own widgets without a way to cheat and embed some web content. I would agree for certain types of apps it may be "good enough" (science, gaming, medical perhaps...anything where the UI is secondary to the 3D viewport)