I would put the difference as that a calculus problem is complex, requiring many different mathematical calculations to complete the full problem and arrive at the answer. One perspective might be to see each of those calculations as different facets of the same problem. I could see how someone could believe it was a tautology.

One could also take the view that a multifaceted problem would be one where different people could arrive at multiple valid conclusions, each facet consisting of an approach that one might take to the problem, as in "There is more than one way to skin a cat." I could see in that sense, that a multifaceted problem need not be complex, and vice versa.

In this way, arriving at a definition of "complex and multifaceted" that satisfies everyone is a complex and multifaceted problem.