I do agree the Flex can work you, rather than you working it, if you get lazy with it,; However, the 3401 has a distinct advantage over the Rupes because you can use the edge, and even the sides of the pad, to get into curves, and for removing deeper scratches easier. Instead of using the entire pad face, tilting the pad makes for a smaller contact area, and removes deep scratches faster than using the entire pad face. This method makes quick work of the RIDS, and also allows you to play with curvature of different areas of every car. I find that there are many curves on a vehicle that immediately don`t come to mind. If you are in the Rupes family, then you probably use other Rupes machines for these areas such as, the iBrid, or the Rupes mini.
I would love to have all the Rupes machines and pads, but I don`t have enough income for that. I find, the Flex can do what the 15, or 21 can do, and much more by being able to tilt the pad.