You can easily alter the script so it's not vulnerable. I have seen that both myBB and PhpBB have MANY security risks just as a forum, without any addons.
My hackforums.net is mybb....one would think if it's so insecure that my site would be hacked. It's targetted constantly by those trying to make a name for themselves. My site has yet to be compromised and I don't have one single mybb core file altered. I stay on top of updates though. Mybb is excellent at notifying and then correcting security problems. I can't say the same for phpbb.
From my personal experience, I prefer MyBB, I've used it on a few websites for about 3 years now and never had any problems with it. The upgrades have always been released quickly after problems have been found and the process has run smoothly. The only current disadvantage with MyBB is that the theme and mod community isn't as large as with phpBB but it is growing and with time I'm sure there will be more.
I prefer myBB coz of its simplicity and more than that, it comes with many features out of the box which phpBB doesn't For instance quick reply and last post in the index. Now i'm waiting for their next big release (1.40)
lets not forget that mybb has a reputation system built in... from copying off of vbulletin even though phpbb still doesn't have it built in.
i think MyBB is way better! But, I like phpBB because it's the biggest database for MODS and Styles and alot of support and active.
I also noticed that installing mods is a snap on mybb, just upload a file, goto admin cp, click enable mod and then change the settings from there.
I like phpbb, although it can be a pain to update and install mods. If you get the seo mods installed from here- http://www.phpbb-seo.com/ it is an awesome script and I am very happy with it.
i am using mybb its seems cool even for simple email validation why we need to add extra mods and time in phpbb