True, it is not a slant-parallelogram type. Like the Campy Rally it has an L-shaped cage with a long lower arm. This makes the guide pulley run up or down as chain is consumed or released. A proper chain length (and an arm ratio of PI, which "they" didn't quite use) lets it track pulley radius very well, at least as well as a slant-parallelogram type. The problem is that the guide pulley also moves up and down with chainring shifts. The ideal chain length for one ring is not ideal for the other. Since the guide pulley must have enough room to negotiate the sprockets while the chain is on the small ring, the big ring always makes the pulley a bit too far away. Fortunately the distance doesn't have to be exact for shifting to work. But it means you are limited to how many teeth a front shift is. With moderate rings a GS or Rally style RD works pretty well. It will not work well at all for a wide-range triple or even a wide-range double. Unfortunately people often want a long-cage RD so they can have wide-range gearing, which often means wide range on the front as well as the rear.