Hi,
I think you misunderstood how IdM solution scenario works. The source of users in personal record in SAP HR. It pushes these records to LDAP server and IdM reads these users from there. So this demonstrates that it's possible to take data from ECC system and export them as records in LDAP. Obviously, it can create/change records.
Regarding limitations/cons. First thing is that you have to build this from scratch. I can't see how you could easily re-use program used by IdM scenario. You can use it as template how to export data to LDAP. There also might be some limitations when exporting to Microsoft AD. IdM scenario uses SAP LDAP server. I don't know, I have never implemented export to AD.
Cheers