1. It is more expensive than S3/Glacier, but they offer "free" read-only snapshots, which can be very useful if your backup software or script does not perform its own snapshots.
2. It can be difficult to determine how much space is taken by the peculiarities of ZFS vs. your own data.
3. Although they offer subaccounts, it is not possible to know the disk usage of a particular subaccount.
4. If you have transfer speed issues, you can contact them and they may whitelist your IP (good if you are doing server backup, not useful for home backup on dynamic IP), which significantly increased speed for us.
5. They have a 10% "free" soft quota, mainly because it is difficult for mere mortals to know in advance how much space your backup will take on ZFS. They send an email when you reach the soft and hard quota.
6. You can communicate with a knowledgeable human by email. This has been helpful to diagnose small but weird issues we were facing.
7. Although we like S3 and Cloudfront, it's nice to have a backup location that relies on standard tool (rsync) and that is outside our usual providers/datacenters.