A full unmerge of affected records is not required in all implementations, and it can have a performance overhead on the unmerge because many child records can be affected. In addition, it does not always make sense to enable this property. One example is when Customer is a child of Customer Type. In this situation, you might not want to unmerge Customers if Customer Type is unmerged. However, in most cases, it is a good idea to unmerge addresses linked to customers if Customer unmerges.
When cascade unmerge is enabled, the child record may not be unmerged if a previous manual unmerge was done on the child base object.
When you enable the unmerge feature, it applies to the child table and the child cross-reference table. Once enabled, if you then unmerge the parent cross-reference, the original child cross-reference should be unmerged as well. This feature has no impact on the parent—the feature operates on the child tables to provide additional flexibility.