Not a zpool destroy, a zfs destroy. Maybe specifically of a snapshot?
You're right for snapshots. zfs destroy can destroy file systems too though.
zfs destroy can, in theory, take a VERY VERY long time(hours/days). But we'd be talking very very very large pools that are very very very busy with extremely big snapshots. I've deleted 1TB snapshots in 20-30 seconds. We'd probably be talking 100TB+ pools easily and the snapshot being deleted was a majority of the pool.
zfs v5000 is supposed to eventually have the feature so that it works in the background and cleans up your zpool when you delete a snapshot. But it hasn't been implemented yet.
As for hibernation, so much hardware doesn't like hibernation I'd never recommend it for a server under any circumstances. Far better for reliability of the server and the services it depends on and serves to do a shutdown. Besides, if you are talking about a pool big enough for a snapshot being deleted to affect a shutdown significantly, you'd also be talking about a server with 100s of GB of RAM. And saving that to disk for hibernation is far less feasible than doing a server shutdown.
Besides, for your average server, what's the likelihood you'd
happen to be deleting a snapshot at the moment of power loss. I'm sure it happens, but its far more likely the server will be idle.