@robertblissitt yup, afterward this seems to be a good best practice...
my hosts were up for 4 month, and because of DNS resolution problem had 77 patches to catch up (80 for one with advanced telemetry enabled)
a rolling reboot would have probably put in front the initial migration/evacuation problem (and subsequent zombies VMs)
and no patches applied, and no pool in a semi upgraded state
note to my future self, try a rolling reboot first.