The kind of codebase you describe with "elaborate type setups" (multiple levels of inheritance, design patterns like Facade used for "decoupling" etc.) make it very hard to read the code and understand what it's doing (unless it's very well documented), and because of that also make it harder to extend without resorting to kludges. Or, if you want to extend it in a way the original designer didn't foresee, you now have to change 5 different classes to be able to access some private property in some class.
