I think also that ZFS supports online rebuild. I remember in my data centre days that a few of the RAID5 products only did rebuilds offline. The HP/Compaq Smartarray stuff was the bees knees though; reliable stuff.
If something particularly bad happened, or you tried being really “clever”, you can get into a rare situation of not being able to import the pool or have it import in read only mode. There are tools to help repair that kind of metadata damage. Then proceed with the normal online repair if needed.