The tradeoffs between Elm and Reason/Fable are that the former is Pure and the latter have imperative escape hatches. There are benefits to both. Elm has a neat time travelling debugger built in and enforces semantic versioning via their package manager. Reason and Fable have much tighter integration with the javascript ecosystem. This tends to lead to differences in community package philosophy with Elm mostly maintaining its own package repo (own vdom system, own date lib, etc) with the occasional ports for interacting with popular javascript libs while the more nascent Reason/Fable communities tend to write interface definitions and call the JS directly (e.g. they use React for vdom).

I group Reason (Ocaml/Bucklescript) and Fable (F#) together because F# is OCaml's sibling language on .Net. They're very similar. The two projects have similar philosophies, specifically to bring their ML variant to the javascript ecosystem in a module-by-module mixed-language fashion instead of having the language be its own island in the browser. Neither has a particularly large community but the Reason guys have been doing a better marketing push this month, at least from my perspective.