One limitation is privacy and abstraction. You can hide implementation details with most ML module systems - eg. hiding the underlying type of `Node`. You can also make local definitions in the module private. It's pretty challenging to get this stuff right - there's lots of research about it.
Also, as noted in other comments, Zig's type parameters are also dynamically typed. This leads to implementation details leaking out. This is not an issue in OCaml and other ML-style languages.
Also, as noted in other comments, Zig's type parameters are also dynamically typed. This leads to implementation details leaking out. This is not an issue in OCaml and other ML-style languages.