It is okay to criticize certain aspects of a previously published work if you disagree with them. Most research builds on what was lacking or imperfect in previous work, so critiquing the assumptions that you think are faulty is perfectly acceptable. However, you should make sure that your criticism is contructive and is backed by strong evidence or a valid argument.
With regard to the language, always use a mild tone that is directed at the specific point that you do not agree with, not at the work in general, and never at the authors. For instance, you could say "The authors have claimed that X leads to Y, but our study shows that this is not always true." The focus should be on identifying what the previous work has done, where your study disagrees with or contradicts it, and how your study builds on it.