This is a very good article that presented a well-balanced view on this topic, must read:
http://daveharrison.net/articles/the-advantages-and-disadvantages-of-content-management-systems[
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To add to it, it is not easy to extend and add your own code to the CMS. Sometimes it leads to conflict between your code and those of the CMS. To sum it up, CMS is general purpose web application which tends to be large and resource hungry, as compared to specific-purpose built and more trim web applications by individual developers.
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