Should a particular backup fail, it is not always possible to re-run the backup within the available backup window (i.e. backups can cause performance issues on servers if run during busy periods). This can result in the backup for a particular day not being available, with the only option to use an older backup for restore purposes.
A further factor is that should a catastrophic failure occur on a backup server itself, all Web/mail servers that made use of that server for backups, will not have any backups until a new full backup is run to a new backup server. Due to the volumes of data involved, this process can take up to 24 hours. It will not be possible to offer customers a backup restore before this process is complete, leaving them with the option to revert to their own backup copies. Fortunately the likelihood of such a failure occurring is extremely small.
Hetzner does offer customers a reliable and cost effective service where we do routinely achieve an over 99% success rate on all our backups.