I use Arq too, however, there are some limitations:
- Scanning of new / changed files takes could be faster and less demanding for the file system (on my Macs, Finder sometimes temporarily freezes when Arq is scanning for new / changed files)
- Mail reports for '0 errors' (all the time, so the mail reports for errors only are rather useless)
- Running in user context only, i.e., if you log out while your Mac is still running, the backup will not continue
- Loading of existing backups (reading / caching index) tends to be slow (and sometimes hangs)
- GUI does not scale up for many existing backups
With that being sad, you listed the major reasons to use Arq and all in all, I am a happy Arq user.
- Client side encryption, with my key (and trust that Arq doesn't phone home). If that is an issue just pre encrypt the data before Arq backs it up.
- Use a storage provider of my choice, i.e. I'm confident that it is secure with Amazon S3 - no unknown shortcuttings.
- Multiple destinations possible.
- Open source tool capable of decrypting the data.