In SchemaExport, the building unique constraint fragment cannot be influenced anymore.

While using an specific dialect, a unique delegate can be provided.
This unique delegate provides a method getTableCreationUniqueConstraintsFragment(Table table). In case of a unique delegate it's possible to override this method, as JavaDoc describes. During SchemaExport, the method is never called with Hibernate 5.0.2. In Version 4.3 the method was called.

Running unit test see under attachement.