When ADB is used as the databinding, all generated code is generated as serializable. In our application, we actually have a SOAP gateway that acts as a proxy to another application and we pass the ADB beans directly between the gateway and the application using RMI. We want to use xsd:union in our WSDL, but currently can not because the generated class extends Union which is not declared as serializable.
If this was just an oversight, then the fix is trivial. Make org.apache.axis2.databinding.types.Union implement Serializable. If the concern is that the 'localObject' value within might not be serializable, then I would either suggest that the declaration of said variable be made serializable or you take a 'who cares' approach to its serializability (i.e. if someone puts something that isn't serializable in the Union object and tries to send it over RMI, then they've dug their own grave).