Storage Box with Borg backup can be setup to work almost identically to tarsnap.

The only real security feature missing is write-only access to the repository (Borg backup in theory supports it, but in practice it's impossible to use it in a way that prevents a compromised host from deleting it's backups - like tarsnap does).

In theory it is less reliable than tarsnap (AWS S3 compared to a single copy on a Hetzner's drive).

Storage Box is significantly cheaper for any kind of real-life backup sizes in my experience.

Borg requires more work to setup and configure compared to tarsnap. There's typically some scripting involved that's unique to your setup and I found that I had more documentation to study before I understood how to use Borg correctly.

A know a few people that have very low opinion of Borg's code quality and stay away from it because of it (I haven't studied it first hand)

You could activate snapshots on your Sotrage Box and don't give your Borg user access to it.