Do not confuse source criticism with the ad hominem fallacy; if certain sources have, in the past, been inaccurate or unreliable, then it is relevant and proper

Do not confuse source criticism with the ad hominem fallacy; if certain sources have, in the past, been inaccurate or unreliable, then it is relevant and proper to point this out and to treat such sources with caution. In this case, the criticism is that the sources -- a collection of popular British papers -- have in the past been extremely unreliable in reporting on scientific issues (example: the alleged link between childhood vaccination and autism), and a humorous comment was used to drive home the point.