There are a few reasons why the Apache Derby embedded database is not recommended for production use. It is more of a tool of convenience that we include so that you can work with caching right out of the box. It is a file-based system that can result in being heavily fragmented without skilled maintenance because caching is heavy on read / write operations. With Oracle or SQL Server or another enterprise DB Engine, you have a more robust built-in suite of maintenance tools.
Using the built-in cache engine also competes with resources on the VDP Server, whereas a separate DB Engine like Oracle would typically exist on its own hardware or even a cluster.
Additionally, with an external database, you have more visibility over the cache tables as you would have direct access to the cache schema while you would not have the same level of access to the embedded Derby.
Hope this helps.