Can anyone think of some good reasons *not* to use an Object-Oriented DBMS to back a website?

An OODBMS is better if you only need to access your data through your objects. If your solution requires additional pathways to your data (e.g. ad-hoc queries, reporting, other applications that need data access but can't make use of your objects), then a traditional RDBMS system is better.

Note: OODBMSes have made a lot of improvement in this area.

I don't know how big your plans are, but the availability of experienced and skilled people to hire (or just to lend a hand) would factor into my decision, as well as just a generally large body of knowledge regarding all of the ins and outs of the DB.

Oracle or MySQL have their flaws, but odds are if you have a problem 100 other people have had the same problem and can tell you how to solve it.

I would say the fact that if you are considering something like db4o that they don't appear to have enterprise examples of powering websites, and are mostly in use for embedded applications.

See my other post on this. (Example websites using db4o)

Nothing technically in the way here, just adoption it seems. However, for speed of development, maintainence and design flexibility, OODBs are pretty unbeatable.

heavy reporting etc. can be done by synching with a relational back end if required which i know db4o supports