Every business I work(ed) with found out that at least backups (on cd, dvd, tape and s3) for some systems were useless when trying to restore them. Sometimes they had been storing useless backups for many years before finding out they were done wrong at the worst possible time.

Nothing to do with the medium, just when you have 100-1000s of systems which are backed up, some of these systems 10+ years old, testing the backups is simply not done in reality.