I love WordPress for my sites and for people who have an idea of what they're doing... but:
A lot of clients ask about WordPress and its strange errors. I hear about a new error every time I get a ticket about WordPress. Sometimes the errors it gives aren't really relevant, though, which can be quite a pain to investigate, although most of the time it is fine.
WordPress is also a quite slow without caching or optimization.
The good thing is that no one has asked about CMS I don't know much at all about, like Drupal or Joomla. At least we're all in the same boat with WordPress.