The McGill student newspaper has a good commentary on the Macleans rankings, although they question not only the elements that went into those rankings but the merits of rankings themselves where there are so few law schools.
Most of the criticism that I have read focusses on two considerations: the ranking of faculty by Canadian citations only, and the ranking of students by Lexpert-ranked "elite" firm hiring. Both are, I think, valid criticisms that probably speak to an unfamiliarity with Canadian legal (and Canadian legal academic) culture.