New Z6 owner here. I can confirm the eye-autofocus is very nice and quite usable. I'm seeing a *much* higher hit rate at 1.4 with the Sigma Art 85, and the 24-70 f/4 kit lens is much nicer than I expected. It is far and away the best kit lens I've seen.
I suspect the Z6 focuses better in low light than my D850, but I have no data to back that up.
Eye AF does *not* focus on animal eyes, at least not on my dog's. In my extremely unscientific test, it finds my daughter's eye almost instantly, but hunts when I try to focus on my dog in the exact same lighting conditions.