What really surprised me, that Facebook with so active usage of ReasonML doesn't push/sponsor Multicore OCaml. Surely that will improve performance in their high-load setups.
ReasonML has only very superficial changes to ML using the ppx AST extension framework. Multicore GCs are not really what Facebook planned on contributing, or at least has shown signs of contributing yet. It is also a bit outside of the scope of their project since they (mainly) market reason for compiling to JS VMs.