You should only specify some preferential details about them that you disliked, rather than things that they just need to improve. Like instead of saying “Your academics aren’t impressive enough,” say “We liked the urban campus of XYZ School” or “ABC School’s students had a sense of humor that wasn’t quite as in line with me as XYZ School.” That way, they just remember that not everyone’s gonna be into the unique things they offer, and it’s not that they’re not good enough of a school for you.