The effectiveInstant requires a higher level of precision (SHALL have at least be precise to the seconds) than a effectiveDateTime, where a profile can address what is actually needed.  This has been defined in data type definitions, providing enough guidance for the implementer to determine when they need the flexibility of level of precision with dateTime, or always require to be as precise as at least seconds.   This could then be further clarified in the profile used for the specific use case where needed.