I agree with all of the above but there are two good dynamics.

For small niche subreddits where you only get a few answers it seems less likely that those are advertisements (and user history is often telling)

And on bigger subs, it's much harder to manipulate the answer unless your product is already pretty decent and you're just fighting over some market spot. In other words, obviously bad products won't be able to advertise this way because of negative comments, but some covert brand building certainly can be done.