The main advantage Flow has over Typescript is soundness. Neither language is sound, but Flow tries a bit harder, and so there are fewer cases where you write bugs that 'should' be caught by the type checker.

Here are a few examples of unsoundness in TS: https://www.typescriptlang.org/play?strictFunctionTypes=fals...

https://flow.org/en/docs/lang/types-and-expressions/#toc-sou...