Thanks. I am ok with accepting the fact USB storage with ZFS is unreliable. I am ok with not using it in real case scenarios.
My point stands however in understanding what broke so I know what to look for and, should I be crazy enough to try something similar again in some use-cases, know what to alert on.
Call me curious. Everybody tells me it breaks, nobody tells me "look, it breaks here, and this is how you can see it".
I will try for another day or two and then will write it down on my notes as "unusable due to bad logging/debugging options", not just because "it is USB" if that makes sense.