Dart is a great language, much better than JS/TS in my opinion - especially now that they are adding metaprogramming abilities. The compiler is solid, devtools are great.

Flutter is fast and will output to pretty much anything (including web, though it definitely does not *feel* like web) and there are tons of libraries in the ecosystem. Including many good native OS looking widget libraries like Material, Cupertino, Adwaita, etc.

Downsides: Less devs than Javascript, obviously. And too much reliance on Google, who could pull the plug. But it's open source and so many companies are invested in it that I highly doubt it will die.