Brandon's podcast, Writing Excuses, has an episode about writing groups and critiquing. In it, he and his co-hosts agree that critiques that contain only criticism are only marginally useful, as well as being hard on an author's fragile ego.
A manufacturer is glad to have succinct reviews of how their product has failed or not met expectations. The designer of that product would not appreciate a random person that got hold of the blueprints telling them without preamble why the product will not work. If you have enough credentials, they will take your advice. If you don't, they will be offended.
The bottom line is that everyone likes to be complemented, and unless you're talking to a very good friend you have to butter people up before criticizing them if you don't want them to get indignant on you. Even then, if you value your friendship you'll probably soften the blow a bit. Use courtesy, politeness, and civility. Even (especially) in text. I'm not saying that your review should be 4/5 praise if that's the rating you gave. A little bit goes a long way, and people usually assume that anyone who starts out by complementing them is alright. But you know all of this already. Carry on.