Steve I am a bit puzzled....SAGenealogy is free to search however the indexes don't not have all the information that is on the CD's however for a small fee they will transcribe the entry from their records.
SAGenealogy filmed the District Registers which were no longer required by the Adelaide Registry after the districts were abolished. You can read the history here
https://www.genealogysa.org.au/resources/online-databases/18-uncategorised/62-south-australian-certificates-story.htmlAs with all paper based bdm records, the returns of which were to be submitted regularly to each colony/state registry, many were lost in transit. I remember reading about one incident in South Australia where the quarterly returns from the Copper Triangle were destroyed in a coach fire. I think one historian advised to search both the district registers and the Adelaide BDM Registry. I can't locate the info at the moment.
Of course you will have those who didn't register their chn's births; ministers who didn't notify the registry of marriages they had performed and funeral directors who buried people and didn't complete their paperwork along with local registrars who misunderstood the need for death certificates if a person's death was the subject of a magisterial inquest. All off the top of my head and someone is sure to come along and tell me I'm wrong

Who of your ancestors 'disappeared'? Have you asked for specific help here on the board?
Must stop rambling....comes with age

Cando