With traditional file systems, the way in which data is written is inherently vulnerable to unexpected failure causing file system inconsistencies. Because a traditional file system is not transactional, unreferenced blocks, bad link counts, or other inconsistent file system structures are possible. The addition of journaling does solve some of these problems, but can introduce additional problems when the log cannot be rolled back. The only way for inconsistent data to exist on disk in a ZFS configuration is through hardware failure (in which case the pool should have been redundant) or when a bug exists in the ZFS software.
The fsck utility repairs known problems specific to UFS file systems. Most ZFS storage pool problems are generally related to failing hardware or power failures. Many problems can be avoided by using redundant pools. If your pool is damaged due to failing hardware or a power outage, see Repairing ZFS Storage Pool-Wide Damage.
If your pool is not redundant, the risk that file system corruption can render some or all of your data inaccessible is always present.