If I set "ldap machine suffix" to ou=Computers (or ou=Computers,dc=mydomain,dc=com), it gets ignored, logs are showing that samba still looks for computer-accounts in ou=Users If I modify the smbldap_conf.pm to insert Computers into ou=Users, joining domains and the like works.
not smbd, but rather nss_ldap. Known limitation in the current design. Marking as fixed later.