I've heard good things about OCaml, but it's extremely niche.
That said, the fact that OCaml is used by Bloomberg and Facebook (on multiple apps and products) alone make it not niche http://www.ocaml.org/learn/companies.html
That plus ReasonML's growing popularity, which is essentially just a syntax extension of OCaml, makes it even further not niche.
And that counts in all sorts of ways. You'll have much more trouble finding answers to any questions you have. You'll have less chances for community meetings or educational content. You'll definitely have more trouble finding new developers for your projects, both in business and in open source.