Your complaints don't sound like they are due to a mechanical error with the controller membranes, but rather a complaint about the functional design of the Duo/Turbo/PCE D-Pad. What is your opinion of the Master System pad, or Genesis/Mega Drive pad? I've always felt like my Turbo controller fell nicely between those two in terms of functionality. While not as glorious as the Genesis/Mega Drive controller, it was not nearly as bad as the master system DPad.
The Sega pads are especially awful for being too sensitive on the diagonals. I can't beat Winger Clinger in Battletoads Genesis, because its nearly impossible to get the natural directions only on the DPad, and that level is especially sensitive. The master system Dpad is the worst of the bunch.