I don't particularly use OCaml anymore (though might in the future I guess), but one thing I remember is that sometimes when you had different parts of type system interact the compiler would just complain that you can't do that (mixing variants and GADTs was one such case I think?) without any real indication that this would be a problem before you try it. I get why this happens and I don't know how many people would run into it, but some kind visible notice or documentation for these cases would be nice.