I know that Rust provides some additional compile-time checks because of its stricter type system, but it doesn't come for free - it's harder to learn and arguably to read
I like to call it getting "union-pilled" and it's really hard to accept otherwise statically-typed languages once you become familiar.
And fwiw I've used unions in typescript extensively and I'm not convinced that they're a good idea. They give you a certain flexibility to writing code, yes, does that flexibility lead to good design choices, idk.
You could create your own Result<T, Error> type in TS but people don't really do that outside of ecosystems like Effect because there isn't usually a reason to.
The downside is that union types require some notion of subtyping, since otherwise A∩B is always empty for distinct A and B. Unfortunately, subtyping is apparently very difficult to implement in HM-like type systems (like Rust and ML) such that it plays well with type inference.[0] Hence, the downside of having union types in a language is that users have to write out types more often.
Unlike kibwen, I don't think Rust's type system is particularly essential for new languages. It's a design choice where one side has more powerful types but the other side has more powerful type inference.
[0] https://en.wikipedia.org/wiki/Hindley%E2%80%93Milner_type_sy...