FlowType has better soundness than TypeScript. I like to think of TypeScript as a "feelgood" typesystem (especially how it handles predicate functions, type variance in function arguments, and type casts). The only downside is that Flow's typesystem is volatile. You _might_ find it painful to upgrade between versions.
With that being said, I still prefer Flow. Not sure how relevant this is, but at my company we mostly write Flow in the teams. For libraries I tent to provide both (code being written in flow, with with .d.ts provided). I also maintain both 3rdparty ts and flow type defs.
TS tooling blows Flow out of the water, though. But, I've heard flow team at facebook is focusing a bit more on DX and on making the typesystem a bit more ergonomic and expressive. :)
This is the dealbreaker. Building Flow in OCaml was a mistake, full stop.
I'm not convinced the level of soundness we're talking about is important except for the people who like to wax philosophical about PL theory. TypeScript has never been "unsound" in a way I actually care about and I would argue if you get caught up in a soundness issue you're already doing something completely crazy.
It's true that ocaml has different contribution entry profile. The only "issue" here is that flow being a js tool, naturally would attract more js contributions, but that's it. You can't say ocaml itself is not fit for purpose because this kind of tool is pefect use case for ocaml. It would be interesting to see flow rewrite in flow itself thought.
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.
I use flow as well at the company I'm working for. I found lightweight setup that works surprisingly well for me. It started as experiment to use types in comments. There is no transpilation phase at all, just pure js with types in comments. All my projects use simple npm test "eslint . && flow check && jest" and there are no prepublish hooks, no manglings, no source maps etc. It means you can edit linked modules in place and get instant feedback. The workflow is very fast and non intrusive. It looks like this [0] or this [1] etc.
With that being said, I still prefer Flow. Not sure how relevant this is, but at my company we mostly write Flow in the teams. For libraries I tent to provide both (code being written in flow, with with .d.ts provided). I also maintain both 3rdparty ts and flow type defs.
TS tooling blows Flow out of the water, though. But, I've heard flow team at facebook is focusing a bit more on DX and on making the typesystem a bit more ergonomic and expressive. :)