Unfortunately speaking plainly and directly in all cases runs into some pretty hard cultural boundaries. What you say and what people will interpret you as saying can be quite different. This will seem patently obvious to some people. I will explain it from first principles because it is not obvious to everyone.
The way the social protocol works around here (US/Canada) is that it is just fine to be directly critical of others if you are talking downward socially. Boss to subordinate, parent to child, teacher to student, it's just fine. But it's not really OK peer-to-peer or upward. And if you insist on being bluntly critical peer-to-peer or upward, what you are saying carries the additional meaning that the person is a real bozo or messed up really badly, so you are justified in talking down to them, as a sensible knowledgeable person dealing with a fuck-up.
That said, it is possible to convey criticism sideways or up, but it requires some social tap-dancing to emphasize that you are only telling the person they are mistaken, not that they are laughably hilariously wrong. The simplest of them is understatement. Compare "That doesn't look right to me. Could you check that it's doing what you expect?" to "No. That's wrong."
Next time you are tempted to be bluntly critical, keep this in mind. Your message may be taken as much more severe criticism than you intend.
