There are also advantages to union types. For example, a Union{T1, T2} can be narrowed to just T1 (by the compiler or the user) without being a breaking change

There are also advantages to union types. For example, a Union{T1, T2} can be narrowed to just T1 (by the compiler or the user) without being a breaking change - indeed, the compiler is free to do exactly that in Julia. In contrast, in Rust, the compiler can't look at a function that produces an Option<T>, decide that in this case, it will always return a Some(T), and then have it return a T instead. That happens all the time in Julia. And also, sum types take more boilerplate code to use than union types. In my opinion, sum types are more useful for static analysis, union types are more useful for ease-of-use.