A little late to the thread, but I had a slew of spammers registering on one forum as soon as they figured out the Q&A. Banning left and right. As soon as I changed Q&A, the registrations stopped cold.
But, recently when I had a
strange issue (resolved by bumping to php7) it revealed the level of attempted registrations on one forum - because gravatar was erroring out, and the server error logs were popping in as many as 5 a minute, all the email addresses were not in the user base, and most of them looked fake. I had 97 pages of error logs in just a few days.
On another XF forum, no spammers really, and none of the gravatar errors, so no attempts either. Which is odd.
Both are using StopSpamForum, Askimet, and Project Honeypot keys, and Q&A captcha, nothing else.