[stoppingdown]

I would not rate the burglar problem too high. Laptops and desktops get them some value - but second hand external drives? Here I would be too concerned about failures than to trust these things any kind of data.

Backdoors might be in the OS of other NAS as well, or worse - if it comes from China, I'd be rather sure there is a backdoor in it. I can imagine a scneario where only one of my machines is connected constantly to the internet, others would be offline (except for updates)

Same for me: I have multiple copies of photos (and other stuff) at home, and if burglars manage in getting all of them, well they'd manage in getting more valued stuff than my photos, and I would be worried for other things.

Backdoors concern me too (even though everything is behind a firewall, based on Linux and that I can finely tune, so I could just block all traffic coming from the NAS). And I share the same concerns about China-made stuff...

[Kunzite]

They're all Taiwanese companies - QNAP, Synology, and D-Link (which makes/made? the WD MyCloud NAS). The difference is, that to my knowledge neither QNAP nor Synology were caught with a backdoor; and they have frequent security updates.
OTOH WD failed to remove a backdoor which was already fixed in the D-Link software. And since their NAS are/were? made by D-Link the argument that it's the same manufacturer doesn't quite work

For photographic purposes I'm mostly concerned about data lost. However - while I didn't tested it... yet! - AFAIK the QNAP backup application supports AES256 client-side encryption (L.E. for Cloud Backup. Other backup applications might have different options)
Just remember to keep backups of the encryption key... if you're paranoid, print it and put it into a safe.
L.E. Actually it's a password, from which the AES256 key is generated. In the event the NAS is unavailable, you can decrypt the files using openssl (and said password).