Not /u/rsync, but I have one of these accounts. The snapshots are immutable (as are all ZFS snapshotss) but you have the ability to run `zfs destroy` on them, so there is a risk there. (When they're doing the snapshots for you, you don't have that ability, but then you just have a filesystem, with no access to the underlying ZFS.)

My solution to the `zfs destroy` risk is to make my backups pull-based, where rsync.net connects inbound to my production server, and rsync.net specifies the necessary commands on the production box to grab the raw encrypted streams. That eliminates the ability of an attacker that is on the production server to run arbitrary commands at rsync.net.

There is still a small risk of data destruction if an attacker gets your rsync.net credentials, but those can be protected via off-line storage and secured workstations, which works pretty well.