from what I know, Flutter is still unusable on iOS because they haven't found a way to fix the extremely janky first load (after install, after upgrade, after reboot) animations.
This problem appeared more than a year ago when Flutter made the switch from OpenGL to Metal on iOS.
Flutter is actively working on the bug you're referring to, and I think you're valid in bringing this critique of the jankyness of animations on first run, but the large number of Flutter apps currently deployed clearly proves it is very usable on iOS. You have to workaround that jank, which is extremely frustrating, but I wouldn't write off the entire platform for it. It will be fixed.