I think robustness is helped a lot by the fact that it’s the production compiler used at Jane Street

Yeah; all the really dangerous extensions are gated behind flags. But there's still a very significant number of optimisations available by default that just work well. I've taken to compiling my normal OCaml code with OxCaml these days to get a free speed boost (but buyer beware: the dependency management can be tricky; I have a giant monorepo to help out https://github.com/avsm/oxmono)