I am inclined to disagree. ordsets is a nice consistent library that could be optimized a lot further, even completely rewriting it using low-level primitives. I’d rather keep this option open. Then, multisets are probably best represented as a list of pairs Term-Count and supported by a proper library.
Of course, anyone can take the risk and use the ordsets library beyond its documentation. A simple assertion/1 call in a directive could be enough to warn that things changed.