When we (darklang.com) were working with Rescript, I was not optimistic about Melange's approach, but in retrospect I think it has legs.
The challenge with ReScript is that the authors really were focusing on the JS ecosystem, and didn't care at all about OCaml. Their fusion with the ReasonML community didn't make a whole lot of sense. ReScript is a much better language now that it has split than it was then, if you look solely through the lens of a better JS.
But, as Elm and other compiled-to-JS languages have learned, Typescript has gotten good enough that other languages might not be sufficiently better than TS to attract enough interest to have a vibrant community. Doubly so since the JS-interop story isn't nearly as good in ReScript as it is in Typescript (though it's much much better than in Elm).
However, the aim of writing programs in OCaml which compile to nice JS and also native OCaml, and allowing both client and server to be written in OCaml, remains valid and interesting (and I would argue was held back by js_of_ocaml, the pre-existing way to do it). So I think there is something interesting here, though it isn't a ReScript competitor except tangentially.
Though I cannot really understand where ReasonML (a JS-like syntax for OCaml) fits into this - if the target is OCaml devs, than surely the ReasonML syntax stuff is not valuable anymore?