It is always safe to use mole method. One should apply POAC(principle of atomic conservation) only when it is mentioned in the question that the no. of atoms of such and such elements are conserved during a given reactions, however in such question mole method can be also applied. The one advantage of POAC is that the question can be solved very quickly but one should rely on this method only when one is adapt in reading between the lines.

"Hence POAC should only be applied when it is mentioned in the question that the the no. of moles of such and such element is conserved or it is given that the reactant is completely converted into product or any statement implying the former".