XML backups fail at a certain size, regularly. We recommend the alternative backup strategy:
http://confluence.atlassian.com/display/DOC/Alternative+Backup+Strategy+for+Large+Confluence+Sites

This is a more effective way to do backups anyways. The xml backup should be a migrate-db-only solution. Other than that, DBAs should be expected to made db dump backups.

This should change in both the documentation and in the product. I am going to change the db setup guides to include this as a standard procedure when setting up an external db. Here's what would be nice for product-level changes:

1) An email notiification should be sent when an xml backup fails.
2) A banner should be displayed in <baseurl>/admin after a failed backup.
3) A note should be added to the hsqldb note about db backups after migration