Yeah. Some existing algorithms are more easily expressed with mutability, and for FFI it's very needed; I think some community-wide messaging regarding this is good enough. Constraining them at the type level might make adoption a bit harder.

We're just a syntax for OCaml; we don't change how it runs. Same GC. Reason's mMuch less sophisticated than you think, but also much fewer unknowns. https://reasonml.github.io/try/

The goal of Reason (and of BuckleScript) is so that people can convince their coworkers that OCaml isn't an esoteric unmaintainable language. BuckleScript is JavaScript: The Good Parts: The Good Parts.