I like to define more precisely:
In SchemaExport creation is tested only. During creation the overridden UniqueDelegate is consulted.
Until version 4.3 overridden UniqueDelegate gets consulted for

• getColumnDefinitionUniquenessFragment
• getTableCreationUniqueConstraintsFragment
  In version 5.0.2 overridden UniqueDelegate gets consulted for
• getColumnDefinitionUniquenessFragment
• getAlterTableToAddUniqueKeyCommand
  getTableCreationUniqueConstraintsFragment is no longer called.
  We used getTableCreationUniqueConstraintsFragment() to build a create statement like

  create table person (
   id ...
   primary key (id),
   unique key (name)
  ); 
  

  I agree, there is a workaround by using alter table,

  alter table person add unique constraint person_name_uk (name);
  

  But the java-doc still says, its possible to override getTableCreationUniqueConstraintsFragment() ("Uniqueness can be defined in 1 of 3 ways...":
  http://docs.jboss.org/hibernate/orm/5.0/javadocs/org/hibernate/dialect/unique/UniqueDelegate.html

At least a clarification in the java-doc is appreciated.