My advice (and daily practice): If IDs are exposed, expose them as strings. If that ID is a compound key of a database, serialize it into a single string. If the ID is exposed via webservice, use URIs. In a entity provided by a webservice (e.g. a JSON-LD document via HTTP), use URLs or URNs. If possible, provide both and a translation service that translates URNs to URLs. URNs should be used for long term storage, URLs for transient use.

If I follow my advice, the type of an ID is an implementation detail of the persistence layer and/or service endpoint.