A few years back I looked around what options exist for building a cross-plattform app. It should support mobile platforms (at least Android and iOS) as well as the web, properly.

The problem with cross-platform frameworks is that they either focus on a good mobile experience, and web becomes an afterthought, or vice versa, meaning mobile basically embeds a browser. Flutter is one of former; back then the web variant was in alpha and basically just a canvas that couldn't even adjust aspect ratio. Since web had priority for our project (accessability, reactive scaling, etc.), this was a no-go.

We ended up going with Ionic (which is basically Cordova with some components on top). The web experience is tweakable to a low level, and the mobile experience is acceptably good, even if not perfect. And we get to cover 3 platforms with the effort of building for ~1.2 apps (there is some platform-specific work involved still).

I'm not even sure if cross-platform is at all solvable in a perfect way, since the UI patterns between mobile and web are just too different.