Having used both S3QL and Tarsnap.. In my experience, S3QL's use of remote mounted filesystems is fundamentally unsuited to unattended backups. On many occasions, S3QL's mounted filesystem would break mid-backup due to network issues, and stay mounted and unable to recover. Then, from that point forward, no backups would ever work again without manual intervention because the mount point was already open.

Additionally, S3QL would periodically issue new releases that didn't support old versions of the file format, or only supported them a set number of releases back. So if you didn't upgrade frequently enough, you'd find yourself with a release that refused to read your existing gigabytes of backup data. And then at that point, you have to do a binary search to find and recompile old releases in the vain attempt to resurrect your data and avoid having to do a full backup from scratch.

Bottom line: Stay far away from S3QL and instead use Attic, Borg, or Tarsnap.

Also, if you select Attic or Borg, check out Atticmatic: https://torsion.org/atticmatic/