This is, flat out, the worst thing you can say to someone who wants to critique someone else's work. You're not only harming the person doing the critique with such a put-down, you're harming the person that made the work in the first place by discouraging bringing up flaws. Everyone has different viewpoints, different likes, etc.; the author is only going to be aware of their own unless others actually start a discussion. Not discussing cons doesn't suddenly make them cease to exist.