My experience with Flutter was that making the UI itself was pretty great. But making the rest of the app sucked. FFI support is bad. Platform channels are miserable. Doing it all in Dart is awful because there's no cheap concurrency - it's the same multi-heap with expensive "IPC" shittiness that you get with webworkers, but way less justified since you're nearly always on platforms with native threads. It's a huge pain to try and migrate piecemeal too, so if you have an existing app/codebase you're looking at a re-write.

UI performance was fine (not any better than native though), but RAM usage was pretty bad, a lot higher than native.

Really fantastic iteration workflow, though.