It being written in OCaml is why the tooling is poor. TypeScript integrates perfectly with most build tooling (e.g. Webpack) because it's just JavaScript like every other tool. To run Flow everything has to be shelled out to an external bin which adds a lot of flakiness and manual configuration if you want to integrate it with your build config.

I understand OCaml is great for writing meta languages (ha) but it's bad for the end user.