Google's John Mueller wrote in a Reddit thread that link spam in forum comments work "very badly." He posted this based on after someone asked if Reddit comments are dofollow. The guy wrote:
I recently commented on another thread regarding this, and I just did a double-take and reddit comments indeed appear to not have the "nofollow"-relation? So are reddit comments then essentially dofollow and SEO-worthy? How do they work?
John Mueller responded:
How does link spam with forum comments work? Very badly.
Then someone responded to John, asking him for a source for that comment. :)
Forum discussion at Reddit.