One way to avoid this harm, while still sharing harsh feedback, is to follow a simple rule: if you’re going to say something sharply negative, say something genuinely positive first, and perhaps something genuinely positive after as well.
I feel this “hamburger” rule seems a little formulaic at first, but I realize it does work. I agree with Ko that it forces critics to find good aspects, even when a design appears weak overall. I know from my own experience that if I only receive negative comments, I will not have the motivation to solve that. However, if that is mentioned well, I'm much more open to hearing the criticism. This approach makes people slow down and actually notice the good parts of a design, instead of just tearing it apart.