When you set the Wii U such as there is no overscan in the Wii U menus (by setting your TV to Full Pixel or Just Scan), when you then launch the original Wii menu or games, there is underscan instead, with a black border all around. Either fix the underscan or allow screen size settings for both the Wii menu AND the Wii U menu to eliminate complaints from people who can't turn off their TVs overscan.
While you're at it, don't output original Wii game material in 5.1 LPCM, because this kills the ability for Dolby Pro Logic to work, hence all original Wii games are robbed of surround sound. Either emulate Dolby Pro Logic and output in 5.1 LPCM, or only output 2.0 LPCM and allow our receivers to decode the Dobly Pro Logic signal so we can get proper Wii mode surround sound back.