My biggest pain point with Rust (in a startup context) is that Rust works really well, until you get to anything related to threading or async.

Yes, the claim is "fearless concurrency" but you'll still deadlocking mutexes and once you're heavily into async you need to start using language constructs that feel REALLY awkward, like pinning, runtime checks like RefCell, and so on.

IMO if Rust could make that whole aspect of the language more elegant, it'd be much easier to scale up to a larger org.