"To address why holding it vertically may have been an issue, is because radio waves don't spread well underwater, but the human body is 60 to 70 percent water, so it can interfere with radio wave paths. Since you hold it both vertically and horizontally, it gets more difficult to place the antenna where the radio waves can easily reach it."
The Product Development department at Nintendo had to deal with this problem, as well as the wall issue. Of course, the team says, working through walls depends entirely on what materials those partitions are made out of. Other factors, such as placing the Wii U on a metal surface, can reduce the WiiPad's range.
One question the team got was if people will be able to use the WiiPad in the bathroom, and for the answer we have anecdotal evidence from Product Development member Toru Yamashita. "By the way, my living room and bathroom are separated by a single wall and I was able to use it."