I have a Raspberry Pi at my parents home (which has r/w to the disk attached to my fathers Airport extreme), it rsyncs every night with the server in my basement (which has all my data on 2 disks.) It also syncs my parents data back to me. It works well but I still need to add a feature to email me if syncing somehow halts or errors out. I use "rsync -av" (over SSH), so nothing is ever deleted.
It could be overwritten though. A good backup protects you from more than just destruction at the primary site. There are various relatively efficient ways to arrange snapshots when using rsync as your backup tool.
Also, remember to explicitly test your backups occasionally, preferably with some sort of automation because you will forget to do it manually, so detect unexpected problems (maybe the drive(s)/filesystem in the backup device are slowly going bad but in a way that only affect older data and don;t stop new changes being pushed in).
Versioning backups seems like a must. Encrypting malware is a thing and has been for a while, just like rm -rf type mistakes which are subsequently propagated automatically to "backups".
Another thing that I do with my backups is making it so that the main machine can't access the backups directly and vice-versa. It is slightly more faf to setup, adds points of failure (though automated testing is still possible), and is a little more expensive (you need one extra host) but to significantly so.
My "live" machines push data to an intermediate machine, the the backup locations pull data from there. This means that the is no one machine/account that can authenticate against everything. Sending information back for testing purposes (a recursive directory listing normally, a listing with full hashes once a month, which in each case gets compared to the live data and differences flagged for inspection) is the same in reverse.
This way a successful attack on my live machines can't be used to attack the backups and vice-versa. To take everything you need to hack into all three hosts separately.
Of course as with all security systems, safe+reliable+secure+convenient storage of credentials is the next problem...
