In my experience the issue with Flutter apps is though they aren't too bad on Android, they have an odd off-putting feel to them on iOS, more than is typically visible with competing cross-platform UI frameworks.
It's because unlike a lot of frameworks like React Native and Xamarin, it doesn't use native UI components under the hood (unless this has changed recently). It's a bit like using a canvas element in HTML and drawing your own toolkit, or old Java GUI apps.