This is my opinion and only my opinion: I think Zope has a DB (object oriented one). For that, I think that DB connectors are for backward compatibility with your ancien software but I would like to migrate data Not necessary. For example what if you want to manage concurrency? You will need row locking and perhaps table locking, thing, which I haven't figured out how to do with zope.
Other point would be the many-to-many relationships. Although there is some product to do this or you can simulate it, I think relational databases are more robust there. Finally, if you use a relational DB, then it will be easier to migrate to another application other than zope. Also, updates are much easier, you won't have problems with the objects as sometimes happens. That's why nowadays I'm working on an application to migrate some of my zope objects to tables. I think the Zope DB is good for documents, templates, scripts, images, files, authentication, and simple objects without special requirements like concurrency, but for more complicated data, a relational db may be more suitable. Regards, Josef