This is basically the same phenomenon that happens with frameworks. At first, the framework gives you a ton of functionality for the (initially) low price of living with its abstractions. As you want more custom functionality, you find yourself either fighting the framework with increasing levels of desperation, or simply coding around it. I'd argue that the best frameworks are such that you can code around them without causing huge overhead or resorting to objectionable tricks.
The same goes for CMSes. Many CMS customers would probably be better off if they took the CMS editor interface as a separate component and wrote the actual page renderer as a custom development separately. The trouble is of course, content systems make it very hard to do that. Having to make things work with the native mechanisms (and the often horrific underlying data model) is what makes huge CMS projects fail or at least perform miserably for everyone involved.
On the very top of the list of abhorrently convolute CMSes would probably have to be Typo3, followed after some distance by Drupal. But the more you work with the initially-liberal Wordpress the more you discover it's not that far behind either.
This reminds me of a bygone era: my startup (now long gone) had its own CMS. Sadly, that wasn't our actual product as we were a pure service company and the CMS was only a tool for us. In hindsight, we should have done it the other way around. Our CMS wasn't perfect, but it shone in a few areas that made the life of both developers and content editors very easy: a simple, accessible data model and an easily extensible page renderer. Unlike many other solutions it was designed to get things done, not to bill a lot of consultant hours. I've yet to come across an open source (or commercial for that matter) CMS that works equally well.