Spam seems to be difficult to control on phpBB, hence, why I have XenForo. Nonetheless, though, I did go thru long stretches of time where I avoided spam on phpBB by using questions as a spam-fighting technique. However, though, possibly things went under with my forums because I didn't change the questions or try a new method.
Also, the URL scheme of phpBB isn't SEO friendly - unless you buy that SEO magic mod for 20 to 40 dollars, something like that. Also, there is a free URL rewrite mod, but it might not work with the latest version.
Oh, by the way, the main things I hate about free forum software is that there are little quarks I've seen on them that can be major problems for visitors. For instance, on one phpBB forum when you quote, the name of who you quoted isn't mentioned. Also, on MyBB and phpBB, you have to edit a post to get the spacing correct. Otherwise, paragraphs won't break (MyBB) or words are too close to quotes.