Fusetalk may be hugely scaleable, but I think it sucks as a forum software.
As far as security flaws in Phpbb, if you are ata all aware, and patch your software like you should, it's a hell of a nice board package for free.
ALL forum software packages have security issues, and like Microsoft, PHPbb gets more exploits than others because of the sheer amount of sites running it.
More copies - better chance to find an old version to hack = pimply-faced asshat gets his jollies.
I won't say it's the best, (vbulletin gets my vote), it does have some issues, but it's not the security nightmare people think it is.