This question is subjective. It all depends on what the core functionality, technology being used, and the clients level of technology competency is. I like rolling out with my own CMS when I can, but I also try my best to have an easy-to-understand tutorial or documentation, so I can avoid hiring a tech support team. Using a third party CMS you can avoid having your own tech support team by referring to their tech support if the client needs it, but you run the risk of limiting your customization that you would envision for the project.
Either way, best way to answer this is for the individual or company themselves. Figuring out what best works for them by trying out x amount of free, privatized CMS and rolling out there own from the ones they like, and figuring out if they can afford and need their own tech support or not.