Barebones CMS is a project that aims to be a flat file CMS that has performance at its core. Depending on your skill level and what you specifically need out of it, the learning curve can be steep from a development perspective. I tried to lower that bar by making videos.
WordPress, Joomla, and Drupal (the "big three" as I call them) are massive, bloated products as a result of over-development. I've personally deployed WordPress in a medium-large customized install configuration in a serious enterprise environment, managing it over the course of several years, and it was an absolute nightmare I care to not ever repeat.
Barebones CMS is targeted at sites that are mostly dealing with static content of relatively few pages but you want to offer a website operator a nice, fast interface to edit that content within a WYSIWYM style editor. While it can be configured to serve dynamic content, that's not its primary purpose. I use Barebones CMS for all of the pages on this very site and use about 90% of its functionality in the website's day-to-day operations.
You'll note that there are very few dynamic bits on this website. The forum software here (it is MyBB under the hood) utilizes several plugins that I wrote and interfaces with other CubicleSoft products to provide a consistent user experience. The bulk of the website is static content, which experiences irregular changes as product updates happen. That makes this site work well within the Barebones CMS architecture.
When I made Barebones CMS almost 8 years ago, there were very few systems like it out there. A lot of changes have happened and will continue to happen in the industry. So there is most likely something out there already if Barebones CMS doesn't fit the bill. Don't try to shoehorn a solution. There are thousands of CMS products out there (some better than others).
The worst case scenario is that you write something yourself. If you do that, I highly recommend using Admin Pack (http://barebonescms.com/documentation/admin_pack/
) for the admin interface, the SSO server/client (http://barebonescms.com/documentation/sso/
) for admin login management, and then Twitter Bootstrap or Foundation or a homegrown template for the user frontend. If you use the SSO server/client, MyBB already integrates cleanly with it via the plugin. You really need solid coding chops to take the custom solution route.
I know this didn't exactly answer your question. You need to think about what you really need from a CMS before finding the right one or rolling your own.