Dart being a GC'd runtime makes it quite hard to use for low latency audio applications, probably not as critical for audio input as output, but still potential issue, for more details see: https://github.com/dart-lang/sdk/issues/46943
Then why bother with Flutter at all?
It's amazing for UI (and portable).
Rust - not so much (or more like not at all) at the moment.
And has a GC running all the time, including when the called Rust code is executing, making it pointless to avoid using Dart in first place.