Portability shouldn't be under-estimated. An ex-colleague had the opportunity to re-write a bad implementation. After gathering the requirements, he had a very nice specification which was portable, lightweight and extensible. Then management wanted to add direct SQL access because old systems had this feature and that made them fast. My ex-colleague didn't want to budge on this issue because the direct access was the reason for the re-write. Management wouldn't budge on the issue either because they were former techies and that's how stuff was implemented on less powerful hardware. Anyhow, the fellow got frustrated and quit. That's how he became an ex-colleague.

Without getting down into the details of any one such case - my inclination would always be to go with the solution that could run on "less powerful hardware". That sort of efficiency has a way of proving to be just that - efficient.

Efficient always trumps cool - unless you come from the developer generation where efficient is the only cool.

"Every single element in a relation (aka table) has to be exactly the same type- no superclasses, no variant types."

Hibernate offers several ways to deal with subclassing. It is also not just dumping objects in the database, like the person in the discussion does, it is mapping the objects to database tables.

True, there may be things databases are not good at (recursion), but overall, in my experience the mapping from objects to database works really well and feels quite natural.