After running "make -j24 buildworld buildkernel" (using GENERIC kernel with no modifications), I ran "make installkernel" and rebooted with "shutdown -r now". I got the following errors upon rebooting.
Solaris: NOTICE: Cannot find the pool label for 'zroot'
Mounting from zfs:zroot/ROOT/default failed with error 5.
Manual root filesystem specification:
Mount <device> using filesystem <fstype>
and with the specified (optional) option list.
(which is equivalent to: mount -t cd9660 -o ro /dev/cd0 /)
? List valid disk boot devices
. Yield 1 second (for background tasks)
<empty line> Abort manual input
I was able to reboot, choose to boot from the old kernel, and everything was working fine as it had before. I didn't see any notices in UPDATING that would indicate any ZFS incompatibilities I needed to account for, so I'm assuming this is a bug.
I waited a few days and rebuilt the source for r364178. It had no problem booting or mounting the filesystem. Whatever the issue was with the earlier revision appears to have already been resolved.