Nintendo Switch is confirmed to be unable to play Wii and Wii U discs, as the hardware obviously lacks the disc drive.
Although I'm not fully certain about every retail and eShop exclusive Wii U game being playable on Switch, the latter is confirmed to only output not two, but one screen at any given time. That omission would compromise some of the Wii U games that isn't only using it for Off-TV Play. Unless they tweak the Switch to support any Wii U GamePad, the Wii U compatibility might not be fully implemented in the final product.
Thankfully, some the most popular Wii U games are either being ported or see their successors coming to Switch down the line. Splatoon and Mario Kart are certainly coming to Switch, as seen in the reveal trailer.