Top 5 Reasons Not To Use Joomla
1) Performance can be slower than desired. In general, coding applications in php / mySQL is faster. Load times can be longer than wanted.
2) Creating custom extensions requires learning the Joomla framework. Editing some extensions can require knowledge of the Joomla framework.
3) Custom templating can be a lengthy process; especially making a design from a psd file.
4) From a web developer's point of view, it can be difficult to alter when you need precise functionality or change its core functionality (like hack registration system).
5) It can be overkill for a small website that does not need too much functionality. It can significantly slow the load time vs a pure php / mySQL cms or plain old html / css.