First you have to understand the difference between a framework and a CMS. Both are software which can be used to create a website. But a framework requires programming to build a website with it and a CMS doesn't. Most frameworks don't have CMS functionality, so the disadvantage of a framework is that you have to write functionality that almost every website needs yourself, such as user administration. A disadvantage of a CMS is that, because they don't require programming, they are mostly big, bloated and therefor buggy (Joomla and TYPO3 are not known for their security).
Banshee is a framework with default CMS functionality. It requires programming to build a website but functionality that most websites need is already there. With Banshee you can create lean-and-mean website in a short time. And because security is the top priority, it's not likely that the result will be insecure, of course unless are you have absolutley no idea what you are doing.
When I was a freelancer, I used Banshee to build
www.security.nl [www.security.nl], the biggest IT security news website in the Netherlands. You can understand that many people will try to hack this website. The website has been audited intensely and no issues were found. That website has been up and running for many years without a single security incident.
I know the documentation can be better, but Banshee is very easy to understand. Although the available documentation is short, it's enough to understand the framework. All you need is proper knowledge about PHP, MySQL and XSLT.
So, if you are looking for a framework to build fast and secure websites in a relatively short time and don't what to write basic website functionality like user administration over and over again, then Banshee is your place to start.