My experience with Flutter was it was only really suitable for UI code. Lack of efficient parallelisms among other things really kills the ability to do non-trivial work. If you happen to be able to stay in Flutter's async APIs then the illusion holds up OK, but otherwise you have to just kick over to native code to do anything interesting. At which point the multi-platform story obviously falls apart.

It's really not unlike the web platform. If you're in the semi-narrowly supported "good" path it's great, but if you hit the limits of things you're pretty much immediately out of options.

And also really bad interop with some really critical components like WebView.