nfs3 clients can write underneath child zfs folder
If you create a hierarchy of zfs folders all shared via NFSv3 and a client only mounts the parent folder it can write to the children folders even if there is a ZFS folder mounted on top of it.
When this happen the zpool will not import properly on a reboot because the child folder cannot be mounted on directory that is not empty.
It also causes failover problems in a HA environment.
NFSv3 should not allow writes in directories that have filesystems mounted on them or write into the child filesystem instead.
NFSv4 either blocks the writes with permission denied or follows the mounts if it has permissions.