I find myself in a better position to explain my action (reason for posting a comment and not an answer on a question) then to argue as a second person. While I've not verified whether it has any effect, I personally believe doing this will make the poster know that I'm reviewing the answer just like an ordinary passerby, hence, the criticism should be considered constructive.

Furthermore, in such cases I prefer giving up the idea of posting an answer usually, so I ask the poster to make required amendments into the answer.