Despite many maturity issues in practice Flutter is the only realistic option for true cross platform UIs that run everywhere. Apart from Qt , but the licensing issue is a hindrance. And yes, the web backend isn't ideal, but it will improve over time.

I just wish Google had built Flutter on a low level core that isn't tied to Dart, so it would be usable from other languages.

Dart isn't horrible and is getting better, but it is still a somewhat awkward mish mash of Java and JavaScript, and I don't really enjoy using it.

The only reason to use Dart is Flutter, which really hurts ecosystem health / library availability and prevents code sharing with the backend.