My only complaints are as follows:
- the dart formatter which is a cancer that wants to do things in only one way. The forced 2px indentation for example is unbearable for me. The maintainers are close-minded and refuse to understand that some people cannot operate with 2px indents. They will even delete your issues/comments at this point.
- Flutter is backed by Google which likes to anihilate, sometimes without warning, projects that don't go as planned. It is open source but still, without Google backing it, it would have never left the ground maybe.