I used to create simple custom CMSs, the function of the CMS aspect of the site was generally limited to customers being able to manage their content, rotate banners, recent news, url rewriting etc etc.
The trouble is as soon as you encounter a customer with rapidly changing needs you kick your self for not using an off the shelf version with a component, or widget in the case of wordpress, for nearly every eventuality.
I find the scope of joomla and the ease with which the administration aspect can be taught is fantastic, the support is second to none and if you are not a developer but have a client with a specific need (and cash) you can out-source to a plethora of freelancers.