1. It has an actually sound type system.
2. The language and standard library are waaaaaaaay ahead of Javascript.
3. The tooling is top notch. Better than JS/TS.
But on the other hand:
4. Way smaller ecosystem.
5. Debugging is worse if you're compiling to JS. The fact that the code you run is basically identical to the code you write in TS can be a big advantage. Only really applies for web pages though.
6. Type unions are way nicer in TS.
7. Non-nullable types interact badly with classes. It can make writing methods correctly really awkward - you have to explicitly copy member variables to locals, modify them and then write them back.
8. Way smaller community.
I know you didn't ask me but I think that not ensuring soudness is a feature because it allows the type system to wrap something that could work without it. Would you like unit tests if removing them would break your code? Maybe it's not a fair comparison, or maybe it is...
But when the types are sound you can use them to compile better code. That's what most languages with "proper" static types (not just type hints) do.
You can use other libraries for this like Riverpod with flutter_hooks and functional_widget which essentially removes the OOP structure of widgets and turns them more into functions, in a way.