You can also split administration up so that, e.g., my friend sending me snapshots can't even log in as root on his backup server.
You rely upon the permissions model not being broken, but once you have a local login, even with limited perms...a large attack surface is suddenly opened, and the nature of attack surface is the odds immediately go up that there is some piece of code running locally that will allow local priv escalation.
Its relatively simple a lot of times to either escalate local privileges, or trick a green admin to escalate privileges for the attacker (i.e. bind-mount namespaces/ebpf).
If you aren't doing a one-way offline backup, it carries the same risks as replication and all the ransomware related risks through rolling/resource exhaustion.
On the sending side `send` is enough, but for tools like syncoid and znapzend `hold,release` are useful as well since typically they hold the latest snapshot on the source which the destination also has so that it can't be deleted on the source before it's used to send an incremental stream up to a newer snapshot only available on the source.