[1] https://news.ycombinator.com/item?id=14238051
[2] http://kaohana.windward.hawaii.edu/2017/05/yelp-extortion-is...
[3] https://news.ycombinator.com/item?id=14238079
[4] https://smallbiztrends.com/2018/01/yelp-accused-of-hiding-po...
[5] https://www.huffingtonpost.ca/2015/01/14/yelp-bullying-pay-r...
Anecdotally, I knew a business owner who complained about Yelp hiding negative reviews on a competitor's page, presumably because the competitor was an advertiser and his business was not. Reading the negative reviews, though, they were _obviously_ shams; it wouldn't be surprising if the complaining businessman had left them himself.
But does what they're doing have to be illegal to be bad? I think most people would assume that if there's foul play, a huge corporation would find a legal way to do it, not an illegal one.