ReasonML has a much more advanced module system via Functors. As far as I know Elm has nothing like this. Despite a fair few arguments over incorporating type classes into Elm, the creator is pretty dead set against the idea.
This does make Elm a lot more approachable, but after a while can feel quite restrictive.
There is bucklescript-tea, which is the Elm architecture in for Bucklescript (and thus also Reason). It is pretty much an exact match, so technically there isn't really anything you are missing out on by going with Reason. In reality the Elm community is more mature which means there is more documentation and libraries available. However bucklescript does interface to javascript a LOT easier - which does open up a much wider ecosystem of javascript libraries.
Agreed, I love Elm but the lack of documentation was a pretty big hurdle for me. Especially because I came from using elixir/phoenix on the backend which I found had excellent guides and docs (for language, framework and most packages).
This does make Elm a lot more approachable, but after a while can feel quite restrictive.
There is bucklescript-tea, which is the Elm architecture in for Bucklescript (and thus also Reason). It is pretty much an exact match, so technically there isn't really anything you are missing out on by going with Reason. In reality the Elm community is more mature which means there is more documentation and libraries available. However bucklescript does interface to javascript a LOT easier - which does open up a much wider ecosystem of javascript libraries.