In short, SEOs and webmasters are noticing tons of links to their sites from this site and it concerns these webmasters mostly due to the unnatural link warnings being sent out to webmasters.
Google's John Mueller went on record saying Google doesn't penalize sites for being linked to by Updowner.com, but rather Google simply ignores any of the links from Updowner.com.
John said in a Google Webmaster Help thread:
Just to follow up here -- for what it's worth, we do ignore the links from updowner.com.
I don't believe I've ever seen a Googler publicly say they ignore links from a site. They might have but I don't remember. I know they do ignore many links from many sites but not sure if they've ever specifically called out a site.
Forum discussion at Google Webmaster Help.