Wii U

There is a kernel exploit currently released for the Wii U that's available up to the latest version (Currently 5.5.1). However, if you're on 5.3.2 or lower, stay where you are as there may be more security loopholes for IOSU userland and kernel.

Without an IOSU exploit, homebrew is currently limited, however, there are IOSU exploits being worked on privately. When a IOSU kernel exploit is released, we will be able to use IOSUHax, which features NAND dumping without hardware, redNAND (just like the 3DS), and debugging software. This will be a good base for hackers to allow them to add more features to the CFW.

vWii
The vWii is what the Wii mode available on the Wii U is called. It is well hacked, just as the original Wii was. Bannerbomb and Letterbomb are not supported, so you will need to use an exploitable game such as Super Smash Bros Brawl in order to use homebrew on the vWii.

Wii U Hacking Thread
http://gbatemp.net/threads/wii-u-hacking-discussion.367489/