The sync between the active and the standby is just to sync any changes in configuration between the active and the standby unit. If it shows "disconnected" it can be a couple of reasons and would be independent of the active unit being unavailable. The sync should be configured to use a different network than the fail over network. When there's an issue with the active unit - the F5 will issue a communications error alert via SNMP to whatever SNMP IP's you have configured. I'd be less concerned with the sync in a failover scenario - though if your F5 isn't set to auto sync, there is always a chance in a failover situation the standby unit won't have the right running config.