Pie menus are somewhat problematic with stylus/finger and a small screen. Because the screen is so small, there are often cases where you can't really display a pie menu to all directions, because you reach the screen edge. And it's really not very good if it sometime would open really somewhere else. At least you lose the benefit of the thing pie menus try to offer in the first place, of learning almost like a gesture to select a certain command. Also, whereas with a mouse cursor you see equally well in all directions, especially fingers block downward directions. Even more, since mouse gives cursor movement and hovering and a touch screen does not, there are a lot of issues with those.