Framework based content management systems

[eluser]WanWizard[/eluser]
Anything is possible.

The advantage of a framework is that you can spend your time coding your application instead of working on housekeeping tasks. How much, depends on the framework.
CI is a light framework, which means that is has all the "low level" functions to quickly build websites, but it doesn't the higher level functions, p.e. authentication & authorisation.

Having said that, there are plenty of third party libraries available to help you solve this.

So far, I haven't found any real disadvantages, other than perhaps the lack of PHP5 features.