My Mac always hangs on shutdown when there are no pools connected: The screen gets black, the mouse pointer is still there and movable, but nothing else happens, even after hours of waiting. I assume a kernel panic is what happened.
I have known this problem for some years now: Can anyone produce it?
I solved this problem by setting up a dummy USB thumb drive as ZFS pool with no data on it that always stays in one of my USB ports. This does the trick.