I agree this may sound like a limitation, a limitation of the OpenLDAP provider we didn’t have on ns6 at the expense of a lot of code. I recall the Samba account database is another account database to keep synchronized with the Unix one (LDAP rfc2307 schema). Very complex setup.
On ns7 there’s just one account DB, being either Samba or OpenLDAP, either local or remote. This saves a lot of code and configuration efforts.
But… we can’t have Samba authentication without a Samba user database.
There should be no big differences between local and remote accounts DB. The only limitation I see on remote account DB is you cannot modify it.