Is there any dancemat that works fine on Wii U?
This foam mat does not suffer from the axis problem, it works perfectly fine on most PCs. The reason they recommend an adapter is because there are cases where the usb port does not provide enough electricity for the pad to properly work. Switching OS-es or changing to USBV1.0/USBV2.0 in the bios helps most of the time. Counterintuitively a low quality usb adapter also helps as it tricks the os into thinking more juice is needed.
That's strange, the pads we sell we use the U-Hid Nano circuit board from ultimarc for anyone that orders a USB control box, the device isn't even supposed to work on USB 1 devices and I have not heard a single person discuss an incompatible device, I think it's because everyone has USB 2.0 PC's even my 2002 PC had USB 2.0 ports and the computer I had before that wouldn't run StepMania anyways I was stuck using Dancing With Intensity or honestly I just used my Playstation.
As for a Wii U dance pad, can't you just use a gamecube adapter, then a gamecube dance pad. We offer multi-console control boxes that have a gamecube port, I have a Wii with gamecube ports so I've never tried playing any dance games on my Wii U yet. Otherwise I'm sure I could get a cheap Wii wand and convert that into a control box for some wireless Wii U action, but nobody has asked for that.
Note, that the axis problem for most PADs can now be fixed here
http://www.stepmania.com/forums/input-adapters-and-controllers/show/483?start=16#post3820
though there are some bugs left, progress has been made
http://www.stepmania.com/forums/input-adapters-and-controllers/show/483?start=16#post3820
though there are some bugs left, progress has been made