What's missing?

In my experience: performance. Even Google's Flutter apps lag horribly in Firefox and feel slow and wonky in Chromium.

The development framework is pretty nice with the way you can share code between native and web, but the end result is just lacking.

Slow scrolling, is a major issue. Hard to solve.

Flutter is great for native iOS and Android though.

The performance is terrible.

I looked at the flutter gallery and it looks like they just compile everything to js files and paint the widgets on a canvas element. Would be interesting to know if they're investing any effort in different backends like WASM. Would probably solve their performance problems. Canvas is obviously a sensible option for rendering but it looks like it might be another reason why their performance isn't great.

They are actually pretty active on the WASM GC group because yeah I think that is the exact plan in the future to ship Flutter web apps as WASM rather than JS.

That's good news. Looking forward to the performance improvements.