Research about frameworks

[eluser]Unknown[/eluser]
Hello!

I'm doing a research about frameworks for PHP.
Currently, I'm researching the frameworks Zend, CakePHP, CodeIgniter, Yii and Symfony.

I'm new in the subject, and I'm reading a lot of tutorials and manuals.
The materials are good, and I could learn a lot of what these frameworks can do.

But, in the moment, I came here trying to know what CodeIgniter *can't do*.
In other words, those features that we miss after working some time with the framework.

Can you please tell me some features that you miss in CodeIgniter for the improvement of my research?

Thank you!
Fabio Schunig