Kopia also supported rclone for a long time, though: https://kopia.io/docs/reference/command-line/common/reposito... . However, in my experience backing up over sftp with kopia can be very slow. I suspect it's unable to use parallel sessions for them (or pipelining, but rclone API probably doesn't do that).
My reason to go with kopia was that previously you were not able to backup multiple hosts into the same repo without great inefficiencies. I'm not sure if they still have resolved that. Another was its native S3 support which I use with ceph.
A perhaps more superficial personal reason is that at least Go is a statically typed language, even if its type system isn't that great..
I was using it for years on an external drive, but then I got a NAS, and did not want to fuss with community packages to get Borg working.
Kopia works fine, aside from the confusing GUI setup process, but it seems to be the least popular up and coming option.
Now it seems that this can directly target SFTP? I wonder what that means for the future of Kopia.