[gentoo-user] {OT} Are "push" backups flawed?

[Grant]

A little while ago I set up an automated backup system to back up the
data from 3 machines to a backup server.  I decided to use a
push-style layout where the 3 machines push their data to the backup
server.  Public SSH keys for the 3 machines are stored on the backup
server and restricted to the rdiff-backup command.  Each of the 3
machines pushes their data to the backup server as a different user
and the top directory of each backup is chmod 700 to prevent any of
the 3 machines from reading or writing a backup from another machine.

I've run into a problem with this layout that I can't seem to solve,
and I'm wondering if I should switch to a pull-style layout where the
backup server pulls data from each of the 3 machines.

The problem with my current push-style layout is that if one of the 3
machines is compromised, the attacker can delete or alter the backup
of the compromised machine on the backup server.  I can rsync the
backups from the backup server to another machine, but if the backups
are deleted or altered on the backup server, the rsync'ed copy on the
next machine will also be deleted or altered.

If I run a pull-style layout and the backup server is compromised, the
attacker would have root read access to each of the 3 machines, but
the attacker would already have access to backups from each of the 3
machines stored on the backup server itself so that's not really an
issue.  I would also have the added inconvenience of using openvpn or
ssh -R for my laptop so the backup server can pull from it through any
router.

What do you think guys?  Are push-style backups flawed and unacceptable?

- Grant