You should always refer to a case or statute in preference to a journal article or a text book if you are seeking to provide authority for a legal proposition. If, however, the legal position is unclear, either because the cases are contradictory or there are no cases on the point, then it may be helpful to refer to secondary authority to bolster an argument that you are making.
Should I criticise the law in answering a problem question?
The object of problem questions is to test your understanding of the law by applying it to particular factual scenarios. However, although the primary objective is not to analyse or criticise a particular area of law, this may well be an important secondary exercise in a problem question where the law is unclear or leads to what may be regarded as an undesirable outcome. While you should not spend large amounts of time assessing the law, it will add to the depth and sophistication of your answer if you briefly identify any such issues and suggest a possible resolution. This is where articles or textbooks may come in handy.