> I also zfs send said snapshots to an off-site remote server, and if I didn't have that capability I'd rsync to an external hard disk or whatnot.
I wouldn't consider a backup system any more that isn't able to establish full integrity of backups. It's also a kinda good idea to diversify the software; ZFS has a really good track record, but if I where using btrfs, for example, I'd make sure my backup software is not the same as my FS.
I do this for my "very important" stuff - e.g. documents and personal photos. That gets copied every year to a third off-site physical drive for safekeeping, and that is an entirely different filesystem. A lot of that stuff is also on google drive for the latest, so I'm not super concerned about the backup frequency there.
The standard zfs send backs up about 25T of stuff I'd certainly not wish to lose, but could eventually mostly be re-created. For my risk level zfs send seemed to be the correct risk:ease of use tradeoff for me. Plus the nice diffing makes syncing a home NAS on a consumer broadband connection not entirely impossible - where even rsync can be difficult for some of my use-cases.
Full integrity of backups is interesting - in a professional environment that means to me full on DR testing (e.g. restore your backups regularly to test them) among other things. For personal use I think automated regular zfs scrubs are about the best you can easily get.
This isn't very hard to do at all. It's very easy to DIY full integrity. Especially with ZFS, where you can take a read-only snapshot and not have the FS changing underneath you while you're traversing it. Here's a previous post of my on the subject: https://news.ycombinator.com/item?id=13129294
Something as simple as the following, done independently for source and destination, and results compared, should suffice:
find -x \
. \
-type f -print0 \
| xargs -0 -n 100 -x md5 \
| sort > /tmp/manifest.source