In Ubuntu’s implementation, root and boot are separate pools (bpool, rpool). Both are (and can be manually) snapshoted. So if boot is corrupted, you roll back. I should say I haven’t tried it though, to see how boot selection works (rolling back rpool is straightforward though).

The boot corruption could occur with the default file system ext4 also, except with ext4 there l is no recourse.

Needless to say, you can always boot from a live USB and mount your ZFS pool (and perhaps roll back).