What You Lose

You cannot create subclasses of data classes. A data class cannot be marked with open to allow it to be extended.

If subclasses were allowed, it is possible that subclasses would change the class definition in ways that might make the code-generated equals() and other methods be invalid. Hence, at least right now, Kotlin prohibits subclasses of data classes.

A side-effect of the no-subclasses limitation is that data classes cannot be abstract.