1. Accessibility is now an afterthought
2. Giant runtime you have to download for every website
3. Bewildering non-native behaviour on mobile
However for the “app” part (after sign in) it’s a one-time download, cached by service worker, only takes a split second.
It would be really nice if flutter could produce a AOT compilation of the “semantic” HTML for the initial page and then hydrate it. Then it’d be pretty similar to waiting for JS bundles for interactivity.
And that's the downside. What would be just two or three nested <div>s in HTML+CSS turns into several screens of widgets, making navigation and debugging much harder. I once had an issue with extra margin in one of Flutter's built-in widgets and spent hours trying to figure out why - without success.