Why would you not have your saves? You can put them on the Wii's SD card if you need to. Not sure how the transfer works, but I know the Wii can move/copy saves on SD cards. It can't load from there, but you can back them up and copy them back to its system memory when necessary

I'd probably have to own a WiiU to give a better answer. And the fact that I don't have one probably means I'm missing something fundamental. Like if it's "transfer" is similar to how the 360 let you change hard drives with the redesign and the transfer cable was necessary and it auto-wiped the old hard drive (which is ridiculous)