Imo, I think that Flutter is one of the best ways to make native Linux applications at the moment. It can look just like a GTK, macOS and Windows app and you have a more enjoyable experience.
Don't know it, but from the website it looks mobile-first and intended for simple apps like Spotify. Don't get the impression that I could build something like Blender or Kdenlive with it. Maybe my impression is wrong, but I would hesitate to use a framework for a purpose that the developers consider fringe.