I just found this little note in the errata for the 11.1-RELEASE:
- [2017-07-25] A late issue was discovered with FreeBSD/arm64 and "root on ZFS" installations where the root ZFS pool would fail to be located.
There currently is no workaround.
What I do not get from that statement is wether this is a rare bug or one that affects many/all installs. So my question to those who already updated a zroot based system: how did it go?