The real downside of using flutter is that you can't embed (or inline) any native widgets like video or maps.
https://github.com/flutter/flutter/issues/73
An awful lot of apps use a map, and a static map in many cases isn't going to be enough. Likewise many apps will want to display video. Looks like they also have an issue open for webview support, which could be a problem too:
https://github.com/flutter/flutter/issues/730
But then I found this, so perhaps there is hope if you can launch it in a new view:
https://github.com/flutter/flutter/tree/master/examples/plat...
Looks like you could do the same for any view, so it's more that you can't have a subview which is a map or video etc, which isn't such a bad limitation (not ideal, but at least it's not impossible).