I just found the .log and .err file
The .log file was a bit terse and it seems like it stemmed from the action: lines when there is a problem.
The .err file only logged stuff at boot time. There was a lot on zed in /var/log/system.log that was not seen in openzfsonosx.*
But the major thing was that it lacked timestamps. I have no clue when the stuff happened. Was it an old problem? A recent?