The problem is depending on the scene. It doesn't underexpose by a fixed amount because if that is the case then it's just a matter of calibration. It depends on the scene contrast and of course whether the it considers the highlight or shadow is important (depending where the area is and how large, of course the matrix metering algorithm is not published and I can't know exactly what it is doing). If the scene is flat then the exposure is ok for either type of film. If the scene is contrasty it tends to favor the highlight and let the shadow goes.
With a large amount of shots where I let the matrix meter totally determine exposure, the slides came out fine (sometimes not what I would think but is good) but a large number of negative film came out underexposed. I have stopped using the matrix meter for negative film altogether.