Explicitly using GenerationType.IDENTITY ... with Database platforms that do NOT support Identity such as Postgres and Oracle does not work.
This results in errors such as the ID value being null resulting in insert statement failing.
The fix is check if the DB platform supports Identity ... and if not ignore that setting.
This is the case now for Oracle and Postgres. Note the reverse check (ignoring Sequence when that is not supported) already existing - for example MySql ignores Sequence generation type.
Fixed in HEAD.