There is a level of "hackability" with any open-source software, as anyone has access to the full source code, and therefore the ability to find flaws and loopholes in it. I wouldn't go so far to say that phpBB is not safe though - the development team keeps pretty current and addresses security holes fairly quickly. As you could imagine though, phpBB isn't as secure as forum software such as vBulletin, which has a $160 pricetag associated with it, therefore making it harder for hackers to obtain the code. In addition, due to the pricetag, the developers seem to have more of an obligation to address bugs quicker than phpBB developers, who are not monetarily compensated.