I have seen situations where dynamically assigned IP addresses for legitimate or new members are shared with ones in the SFS database. Same with user names, if a common user name like "John Smith" or something generic like that. Usually when these are flagged, they go to moderation and can quickly be affirmed or denied.
That said, for the most part I have found the SFS does a pretty good job at halting spammers, with Akismet less frequently but with near-equal effectiveness.
There is an Moderation indicator (a red flag) at the top of the XenForo screen of all XenForo administrators, which has a queue of pending moderation requests. The StopForumSpam, when it identifies a questionable post or member registration, shunts the post or new member request into the Moderation queue, and indicates it with the red flag at the top of the screen.
As an admin, you go into the queue, and either assign or resolve the issue as needed, and then it closes the request.
Sorry for not being more clear, @gerryvz. I've been a XF admin since 2015 and am well acquainted with the moderation queue. What I don't know how to do is pass judgement on someone who shows up in the queue as a result of SFS, how you look at those numbers and make a decision. @Brogan was nice enough to explain, just thought another explanation would help me understand. This is the first time I'm using SFS, and therefore the first time I'm seeing these people in the moderation queue.
The numbers that @Brogan mentions are a huge indicator.
I have my StopForumSpam settings locked down pretty tightly, so that even 1 "hit" sends a registration to the moderation queue, and two hits rejects it outright. And hits reported within the last 14 days. This helps get around older IP addresses or user names that may have triggered the SFS database in the past.
I have found the SFS rejections to be pretty accurate when I examine them.