Always phrase things in a positive way. No matter how good or thorough your review is, phrasing issues in a positive way and pointing out good things will make discussion about the code so much easier, and also obviously make the human interaction at work nicer.
Also there may be novice people or people who just didn't do a very good job with their pull requests. Don't judge. Remember that you are not grading an exam, but reviewing is a cooperative task.