So many variables to consider when offering "constructive criticism" indeed. I guess so much depends on your relationship with those receiving the criticism and the place (online/in person/email etc) the criticism will be offered. You are right of course, offering insincere positive comments may not be doing that person any favors. I do my best not to offer "phony compliments", tend to just say little or nothing when I see something that does not look good to me. I remember this stern advice the parents would give when we were kids. And, I must keep in mind, everyone's taste (especially in heels) is different, beauty is in the eye of the beholder....
In the case of guys in heels, one must also consider where a fellow is on his heeling journey. It is hard enough for a guy to muster the courage to go against idiotic societal norms in order to explore fashions he has only secretly admired in the past, I tend to err on the side of encouragement in cases like this.