Some were saying they don't trust AW Stats, only wanting Google Analytics. Anyway, what about the stats on XF 1 and 2? Is it trustworthy? Also, if you had MyBB, phpBB etc. and your posts had big numbers, well, is that visitors or bots?
Firstly, it doesn't take into account people who may be using Adblock (apparently, adblock has waged war on Google Analytics) and those who have JavaScript disabled.
Secondly, webmasters are trying to reduce their reliance on Google, particularly with all the fears regarding privacy when it comes to the tech giants. Even if they don't care, their users do.
Unfortunately, that does mean that you have to trust the webmaster to some extent, so you probably want to go for sites with good reputations. As for forum statistics, post counts are absolutely worthless, it is biased towards forums with a long history regardless of traffic and activity.
Now that I'm into Facebook Ads - I wouldn't even bother with banner stuff. It's just too untrustworthy. Also, off the list - are those sites selling 10,000 visitors for a dollar (pop-under traffic etc.)!