Dart and Flutter are totally fine, and great in production. I really don't understand the hate.
Don’t get me wrong, I would really love Flutter to be THE solution to the cross platform mobile app problem, but sadly it only gets you 95% there and if you really care about the last 5% you’re out of luck.
I believe it’s good for knocking out an MVP if the mobile app is not critical to your business. I would not choose it otherwise though.