Flow is quite good, but over half of our React codebase is components connected to Redux or Redux Form and unfortunately there are no Flow types for the versions that we use, so type inference is very weak at some points.

Besides, Flow has a very ambitious target of delivering types for APIs which were not written with types in mind at all. I thought I'd be able to recreate some of the Elm's type safety with Flow, but that's just not possible and I doubt it'll ever be. But then again, these two projects have very different objectives.