Thirded! I've been building out all my Debian boxes with ZFS root (and everywhere) for a year or more. Not only does this require installing via debootstrap, but there are some subtle things that have to be done just so or the system won't boot. Stretch improves this over Jessie, but it's still clearly not a first order citizen.

Like with non-free firmware, and other such things, I get the licensing and taint issues, but it would be good to see this evolve and be supported in the installer, someday, if only on a "forked" installer like with non-free firmware.