From a comment in HHH-8217. These are no longer true:

http://docs.jboss.org/hibernate/orm/4.2/devguide/en-US/html_single/#d5e347
1.4.1. Customizing the mapping files
The unique-key attribute groups columns in a single, unique key constraint. Currently, the specified value of the unique-key attribute does not name the constraint in the generated DDL. It only groups the columns in the mapping file.

http://docs.jboss.org/hibernate/orm/4.2/manual/en-US/html/ch21.html#toolsetguide-s1-2
21.1.1. Customizing the schema
A unique-key attribute can be used to group columns in a single, unique key constraint. Currently, the specified value of the unique-key attribute is not used to name the constraint in the generated DDL. It is only used to group the columns in the mapping file.