I'm not a lawyer but one must always respect libel laws. So if you plan to dis someone I suggest you have solid evidence just in case a server rings your doorbell and you find yourself in a lawsuit.
Being a writer does not entitle one to break the law.
Your freedom of speech allows you to express opinions. One should actually state that it is an opinion. So it would be legal to say "in my opinion so and so is a thief and a fraud artist". That is different than saying it without the "in my opinion" part.
The same with a product. You may express how you reacted or how it worked for you. But you can't just say something is crap... except with the "in my opinion".
And that's my opinion on that.