The right tool for the right task
The power of business rules management systems is that they allow the business logic (business logic, not all logic) to be exposed to the business owners in a controlled way. The problem with using code to represent this kind of logic is that it will probably never be right - the business users can't read it to verify it and it can't be changed quickly when they need it to so it tends to drift out of alignment.
Yes you can get into trouble with business rules, yes you can expose too much to business users and put too few controls in place. But remember what you are comparing it with - code for which there is no-one who both understands its business purpose and can read it.
Of course, I'm a fan so I would say that wouldn't I...