I think one of the concerns with the MPL is that the majority of schemes which I have seen involve Airbus types. Whilst I don't want to debate Airbus vs. Boeing etc. I believe that the PNF cannot "feel" the control inputs of the PF, indeed from what I'm aware pilots can make contradictory inputs which cancel each other out unless the "takeover" button is pressed.
Once or twice in a K13 glider, the instructor said that I overcontrolled slightly, he could feel the control inputs I was making and hence something that could have otherwise become a bad habit was resolved early in training.
I think the significance of this is that since a lot of the MPL training is done in Airbus simulators, it must surely be difficult for an instructor to spot minor concerns early in training which could become significant over time. Slightly worrying when the airline can only pick from a narrow demographic of applicants (those who can fund the training) if you ask me.