Datastore synced with Rclone broken?

while I don't think that using sshfs is a great idea, I'd like to know if, network wise, a pbs sync is as intensive as a pbs backup. After all rsync is quite good in transferring only the changes.

again why?

Because it's simpler and because the distinction between the vm/lxc and data is somewhat blurry (do you consider a docker container data or part of the os?).
I don't see the problem with needing a working PBS: yes, it's possible that both the PVE machine and the PBS one break at the same time, but that's what the off-site backup is for.
 
It's possible, that doesn't make it a great idea.
Regarding PBS sync only transfers chunkss not already present on the target