As far as is known at this writing, all functionality except nested transaction support and cursor-puts supported by the BerkeleyDB backend is supported by the CL-SQL back-end. Concurrency and transaction atomicity have not been stress tested well for the CL-SQL based system.
Additionally, it is NOT the case that the Elephant system currently provides transaction support across multiple repositories; it provides the transaction support provided by the underlying repository to the user in a per-repository basis.
The PostGres backend is as currently employed is about 5 times slower than the BerkeleyDB backend. This could probably change with continued development.
CL-SQL supports a lot of DBMS systems, but only PostGres has been tested.
The SQL back-end extention has only been tested under SBCL 0.8.18.
The SQL back-end is as easy to use as the BerkeleyDB back-end. However, the multi-repository version somewhat complicates the underlying persistent object management.