There are, basically, two types of CMS.
Firstly, the 'full service' type like Wordpress, Joomla and a hundred others, which allow you or your client to create and modify an entire website 'automatically' (haha!), complete with complex menus and layouts, with all the code included, and all manner of ridiculous complication and confusion.
Secondly, the 'just edit a few pages' type, like CushyCMS and a hundred others, where you provide the website and just 'plug in' the CMS facility. This can work well if all your client wants to do is change the text and pictures on selected pages.
If it is appropriate, the second type, like CushyCMS, is infinitely less complicated for everyone concerned - both you and your client - and will avoid the steep learning curve required for all the 'full service' CMSs, and avoid the never-ending upgrade cycle of these CMSs and their various (often incompatible) plug-ins/themes/extensions and whatever.
You need to analyse very carefully EXACTLY what your client will ACTUALLY modify themselves, if given the chance, rather than what they instinctively THINK they will need to be able to change. Because very often with clients who choose the CMS route they seldom change anything at all in practice. Ever! Often not even a word of text, let alone new pages and menu items etc. In which case a complex CMS is complete overkill, and just a pain in the neck to maintain.