Just use ZFS. It works.
ZFS has bugs like any other codebase, and metadata corruption will make your whole pool unrecoverable. RAID-Z and block checksums won't save you from that.
I speak from personal experience with ZFS. Backups are important, no matter what filesystem you use.
If you're doing differential backups on external USB disks (generally of dubious reliability), you definately want some integrity checking. Bitrot happens!