UCS is good at offering several authentication services (LDAP, Kerberos, AD/Samba4, SAML) for a central user database, has APIs to both automize user workflows based on informations provided by HR systems and integrate / provisioning other systems (like databases etc.), scales do to integrated multi-server-support -- and is fully open source and free. See https://www.univention.com/pro...

I've seen Samba4 productive since several month (based on Univention Corporate Server). It was stable and easy to use even with a few hundred clients. Thanks to the Samba-Team for the good work!