Many people begin their criticism with sincere praise followed by the word "but", which signals that the criticism is about to begin. This may make the listener questions the sincerity of the praise. Use "and" instead, and provide constructive advice rather than criticism. this is possibly the most effective ways to address an issue in written form without seeming false in your praise.
Dale Carnegie, How to Win Friends and Influence People
communication-skills, criticism, positive-outlook