Amazon reviews could potentially be salvageable. The way I see it, their problem is that items sell more when they have a plurality of reviews, so there's a disincentive for the platform to take down positive reviews (because they drive revenue). But Amazon is a platform that depends on repeat business (hence the existence of Prime) so it also has an incentive to ensure that the totality of a product's reviews give a useful idea of the product's pros and cons.

On the other hand, sites that let you remove bad reviews or showcase good ones are running a 21st-century protection racket.