I think one of the drawbacks is the complexity involved in developing a decent one size fits all CMS. There are specialized versions out there, but so far I could not detect a coherent way to approach this. And last but not least, what should a CMS accomplish. What is the basic functionality that is common to all. And what could be added to it at a later stage. Beside the technical backend it is the user interface that matters most. And as Simon put it rightly, we have a good backend, but an aweful frontend.
Now, I think, if all the people out in the community who need a CMS within the next 6 months could agree to work on specs together and come up with a coherent spec something can be accomplished, that will come at a fairly low price for each of the parties and is generic enough to be used in multiple ways.
