It appears Linux root-on-ZFS is a mess. No standard way to do it. Kernel updates require recompiling ZFS. Boot environments are a cornucopia of constantly evolving hacks.
If you're actually using #ZFS on root, on #Debian, what's your preferred hack to make that happen? #sysadmin
I suspect #openzfsmastery might need to assume root on extFS and data on ZFS, leaving root-on-ZFS for the advanced user or a terminal chapter. 
@mwl The boot environment for Linux has always been a mess of ugly and constantly evolving stop gap measures. If you want some fun of diving into this rabbit hole, check out the Linux from scratch stuff.
But yeah, that is far from the only disk related mess. Frankly everything disk related and booting basically is.
If you want to run screaming, check out the partial software RAID implementation md.
IMHO the whole disk subsystem needs re-writen from scratch to work akin to how GEOM does on FreeBSD.
Michael W Lucas 
VVelox