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).

For example, multicore OCaml is not free of race conditions. The GC, while super efficient (pauses are in the milliseconds), is not suitable for hard realtime.

Still, where absolute max performance or realtime are not required, I'd choose OCaml as it is elegant & a pleasure to code in (personal opinion, ymmv).

Poor windows support, confusing and buggy tooling (yeah really), mediocre documentation, global type inference, weird obsession with linked lists leading to performance gotchas, difficult syntax (yeah really), small community.

I can expand on any of those if you disagree with them.