An anti-testimonial is truthful, balanced and critical. It begins with a doubt or objection that is then resolved. It may also include a wish for feature X or Y, or some minor unresolved complaint, as you’d expect when soliciting product feedback. Finally, anti-testimonials tend not to be published on the product/service website.
Anti-testimonials only work for valuable products and services; that being the case, they work much better than normal testimonials (100% flowery praise).