The most frustrating part of negative SEO or any link cleanup is that Google doesn't tell which links they think are spammy links. If you are working for/with a site that has been online for a while, they will have thousands of crappy looking links pointing to their site.

Scraper sites pick up content from other websites all the time and cross post with porn and other crap. Directory submissions that were perfectly fine 5 or 10 years ago can now be viewed as "low quality" by Google. Blogs that used to be well-maintained have been bought by spammers and SEOs and are now content farms.

Google's lack of transparency when it comes to links is very troublesome. I understand they can't give out the ins and outs of their algo, but asking your average website owner to go through 10k+ backlinks and identify and remove the links is a pretty tall task when there isn't a clear standard on what is bad and what isn't.