Mothers teach their children, "If you don't have something nice to say, then don't say anything at all." And even if Mom didn't teach it, the norms of social interaction usually dictate it. Although it's always polite, a compliment may or may not be honest. But a criticism is rarely dishonest. People who like the effort have no real incentive to lie to you by telling you they dislike it. What would be the point?
Most often compliments tell you what you want to hear. That's the echo chamber effect. But criticisms tell you what you need to hear. And the person who levels the criticism goes out on a limb by doing so. That's why they deserve a sincere thank you for the chance they took. Without them, you would have never known what might need to be improved. You would have only been told what didn't.