This answer recommends a particular hostel search engine website. For reasons I explained in a comment, I think this is a bad website (not useful, claiming to be complete but has 0 of 4 nearby hostels I know). That doesn't make the answer bad per se, but it does affect the usefulness of the answer, and should an alternative answer recommend a site that is more useful, I'd upvote that one. The question is, should I downvote the answer based on the quality of the website it recommends?
IMHO the answer is helpful and shouldn't be downvoted. Yes, it's probably not the best hostel search engine and has only few hostels listed.
If you don't find the link is useful, just don't upvote it, but downvote would be too hard, otherwise you should upvote the best answer and downvote all other answers as they're less useful.