Having string slices can enable functions like String.trim, String.take and String.drop (the latter two are not present in the OCaml stdlib tho) to work in constant time which is pretty neat.
However, in GC’ed languages, there’s a common problem with slices where a tiny slice (even 1 byte) to a huge string/bytes array prevents GC from collecting the entire huge string/bytes array. Therefore, the slices abstraction unfortunately leaks and requires careful thinking about memory usage in each case.