Isomorphic codebases. I would much rather use Rust on the back-end than ReasonML/OCaml.

Why's that? Ocaml has excellent performance and is a lot higher level than Rust. I personally see Rust as a language that really only should be used if utmost performance or hard latency requirements makes GC'd languages unsuitable. Most backends for web-app would run only marginally better with Rust than Ocaml and incur a steep productivity and maintenance penalty.

A lot of people seem to be suggesting Rust as a GC language replacement instead of a C/C++/asm replacement, which I've never really got. Is the mutation/lifetime/pointer management really worth it for this kind of thing?

I think it stems from the root cause that many equate GC languages with heap allocation everywhere, and never got to learn properly how to do value allocations on the languages that offer language or library features to do such allocations.

I was wondering about that too. OCaml has a really good track record and the whole Bucklescript ecosystem also proven to be good for reliable frontend development.