ISPConfig supports distributed authentication, LDAP is not the only way to handle this. ISPConfig uses mysql as central authentication source. The data can be either replicated trough mysql master > slave replication or a mysql cluster or trough the builtin replication in ISPConfig.
All changes in ISPConfig 3 are stored in a transaction log and can be rolled back or replicated to other systems. ISPConfig also supports to configure stand by systems with identical configuration.
ISPConfig 3 is completely modular, based on plugins and events. If you want to have your users in ldap, just write a plugin for it that listens on the user_insert and user_update event and inserts the users into ldap then.
You should have a deeper look into ISPConfig to fully understand it before you judge that it is not enterprise ready.