I know Wt is the most stable of them, but it's a bit uncomfortable to use.
CppCMS sounds good but how stable is it? How secure is it?
I have encountered C++ Server Pages as well but there's nothing about their security in there.
Has anyone had some experience with any of those libraries and can enlight me?
First of all, several differences:
Stability, I can tell only about CppCMS, it is stable, and there are applications running it 7/24, the authors blog and the Wiki with documentation of CppCMS are written in CppCMS. So, there shouldn't be major critical bugs.
Disclosure: I'm developer of CppCMS.