I'm not a native speaker and really have to think what and how I write. And I find a 'XY is wrong.' much easier to understand than a substitution that's intended to not hurt somebody's feelings. 'You are wrong' is a simple and clear expression, that of course should be followed by an explanation of What and Why.
If a questioner has problems with being told that (s)he is wrong, I don't see that as a problem of the answerer.