I don't think there's any comparison to be made. The term "fake news" was primarily popularized by a guy who was blatantly lying and who's grasp of the English language barely extends to 5 letter words and who doesn't even really bother with the grammar. "Fake reviews" are literally just fake reviews, it's not even really a technical term, it's just what they are.

I think it was more heavily popularized immediately after Wikileaks' DNC email leaks by the people who opposed the guy you're talking about. Then, the guy you're talking about adopted it as a counter-attack in order to devalue the term. His campaign turned the weapon back on its source. If you were paying attention to how the term "fake news" evolved, it started as a weapon for the left and then was adopted by the right. It's a great way to censor or discredit information that is inconvenient for you, regardless of your political leaning.

https://trends.google.com/trends/explore?date=2015-05-01%202...

Fake reviews are like spam and troublemakers at a comedy club. Nobody but the assholes themselves likes it. Clubs that don't throw out hecklers or fist fighters will fail when people go elsewhere that doesn't suck. There is a clear definition of non-genuine reviews: people who have never tried the product, they are being paid for the review, or you are the seller posting under a sock puppet account.