Why is it not used my many: Banshee focuses on security. Not many care about security. They just want a shiny, eye candy click interface. Hence Wordpress' popularity. And also because most people don't have the knowledge to judge the quality of a framework by themselves. They just follow the herd. Too bad for Banshee that the leaders of those herds like other frameworks.
Advantages: secure, fast, clear separation between modules (like CMS) and the framework itself, clear MVC, many ready to use modules
Disadvantages: none