IMO, the problem of Flow is: not everyone use type annotation. While using ELM or PureScript, the libraries basically be forced to have type annotation. For example the Rx 4 doesn’t have type annotation, so if I want have type safety then I need to “declare” the type.

And as a guy knowing how to program in Haskell, I think Flow’s syntax is not pretty. However, I still choose to use Flow, because this is the easiest way to bring the type safety to front-end (React Native in my case).