I don't know the advantages, but a big disadvantage for me is the lack of Linux support, whereas Flutter has a steady growth in this field. Well, the last time I checked at least, this was my perspective.
Or maybe I'm confusing with MAUI
First line of the linked repo's README:
> The Uno Platform is a UI Platform for building single-codebase applications for Windows, Web/WebAssembly, iOS, macOS, Android, and Linux.