Boot blockers

Speculative execution vulnerability mitigation

If you have an AMD processor (such as Phenom II or Athlon II Regor) this mitigation can cause bootup trouble, possibly in /etc/sysctl.conf or /boot/loader.conf:
vm.pmap.pti=1

Solution

Change the '1' to a '0' in the above. This can be done at the loader prompt with set vm.pmap.pti=0 or in singleuser or after successful boot, remove or revise the line in one of the above files.

Speculative Execution Vulnerablities at FreeBSD wiki

ZFS boot vfs root

Including the following in my /etc/rc.conf prevented successful boot.  Perhaps the setting was wrong. vfs.root.mountfrom="zfs:zroot"

Solution

It seems that this line has an inaccurate location or the indicated zfs location is not set as bootable, this is needed for GPT boot but can be used for legacy MBR as well.

Correct the bootability.
zpool set bootfs=zroot zroot

FreeBSD forum post on Root-on-ZFS boot problem
GPTZFSBoot at FreeBSD wiki