It's much harder to write a review for a product that disappoints me, but it's important to both be honest, and to give a product every possible chance to work.
When I review a bad product, as defined upthread, I don't pull my punches. If I honestly think nobody should buy the thing, I explain why in detail, but I also try to include every tip possible for salvaging it, assuming that someone who already owns it might read the review and appreciate an idea.
For example, on a toy with awful seams, I discussed sanding the seams. On a toy that was unusable, I discussed salvaging the electronic parts for use in a project (the controller to a bad toy is currently serving as a dimmer switch on a lamp in my bedroom.)
And good products that just don't work for me personally -- like a Liberator shape that failed me -- I describe the product with as much detail as possible, both good and bad, and then go into how it failed me, who else it might fail based on my experience, and who will likely find that it works just fine (thin people without memory foam mattresses, in that case).
Or, short answer, yes, it's much harder to write a review when a product isn't the best thing ever.