I meant "only downside" in terms of how to design the API parameters and return values. I did not say it is the caller's choice nor that it is equally reliable

I meant "only downside" in terms of how to design the API parameters and return values. I did not say it is the caller's choice nor that it is equally reliable. It is a system composition decision. It is a system that is sometimes distributed and treated as such from a reliability point of view. Sometimes the actual implementation is more reliable and faster.