Yes. Again, we have this cloud hanging over us. The Supreme Court created a new kind of concept—Mr. Kenney alluded to it—of the architecture of the Constitution.
It's a little hard for those of us who parse every word of Supreme Court decisions to tell you, “Well, here is what ‘architecture’ means”. But my guess would be that if someone said that having two kinds of MPs changes the architecture, they might get a case up, and it would be embarrassing to have a new electoral system under a constitutional cloud.
I don't think you'd have that problem with STV, but I think there would be an argument about constitutionality with MMP. That's all, and that worries me.