Any of them have their vulnerabilities..... so you have to stay on top of the security issues if your going to use one. Postnuke, and PHPnuke have a large userbase so security issues are usually fixed rather quickly. As long as you keep your code updated with the patchs as they become available, it is fairly secure.
Linux Mandrake's MandrakeClub.com uses phpnuke..... Seems fairly stable, even with the tons of hits it gets everyday.
Here is a good place to find other systems.....
http://www.hotscripts.com/cgi-bin/se...=cms&catid=all
HTH's some