The ecosystem. The language is lovely, but dune/opam is not up to the standard of the Go or Rust build systems, and the set of useful libraries is somewhat skewed. Whenever I write a program in Caml, I gain an hour thanks to the nice language, and then lose two fighting with dune/opam.

There's also the support for concurrency and parallelism, which has started to improve recently, but is still years behind what is available in Go (but still better in my opinion than what is available in Rust).