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)