We seem to think being nice means never voicing a critical word. But it’s NOT nice to never give feedback that will help someone improve. And women in particular seem to have to make sure they never say something that sounds “not nice”. We have to couch everything in “I wonder if”.

You are spot on: what ends up happening is the other person hears wishy-washy feedback and resists changes that would improve the work, and may even come to think that all design choices are subjective.