ZFS encrypts most metadata.

Metadata not encrypted: Dataset / snapshot names, Dataset properties, Pool layout, ZFS Structure, Dedup tables

ZFS encrypts: File data and metadata ,ACLs, names, permissions, attrs Directory listings,, All Zvol data,FUID Mappings ,Master encryption keys ,All of the above in the L2ARC ,All of the above in the ZIL

For most uses and use cases this is net increase in security. You can do some operations on data without needing the keys.