If there is a repair/resilver operation running is it safe to reboot the system?
Yes, it is safe to do reboot the system during a resilver operation.
The resilver will pick back up the repair where it left off after the system comes back up.
If the reboot brings any "FAULTED" disk devices back to a healthy state, then they will also begin to repair/resilver given available throughput.
Note: The resilver is throttled by default to be a low priority process so as not to affect client side I/O requests with undue latency.