Flow's type system is infinitely more powerful. It's proper static analysis. Tern, as an example, can only do so much.

JSDoc is verbose, not inline, not directly associated with the code and not very friendly to use, as well as having a large maintenance overhead..?

I mean... that all applies to JSDoc/JavaDoc syntax before I compare it to Flow anyhow.

Flow's comment system is inline - right next to the code you're writing. It's very clear exactly what it refers to and it's easy to use. Plus it's not TypeScript: it's JavaScript, just with comments.

TypeScript is great, but it is what it is and that is not JavaScript (a superset, sure, but C++ is a superset of C, so what?)