An example of union types: many API in JS accept HTTP verbs as strings. With union types, you can say something like:

    type HttpVerb = "GET" | "POST" | "PUT" | "DELETE";

But you can also do more complex stuff:

    type User = {name: string; password: strings} | OAuthID;

By themselves they aren't hard to understand or use.

As an aside, compared to sum types, you don't have to define them before using them: a function can return number | null while in language with sum types you usually define option first and then return option<number>.