From the Intranet Focus Blog:

A CMS is probably the most complex rollout an organization will manage. Even a change to the desktop IT environment is less of a challenge. Indeed, only organizations that have implemented an ERP application or a document management application are likely to have had any related experience.

It is a huge process that requires lots of buy-in and support from around the organization in order to succeed. For membership associations, such as where I work, implementing a CMS is very similar to implementing a new association management system and comes in a close second as far as degree of difficulty.