An anonymous source working on a Wii U title said the hardware wasn't as powerful as he expected, though.
One of the most common complaints heard about the Wii when it debuted in 2006 was that it wasn't as powerful as its brethren. The Xbox 360 and PS3 both boasted a huge amount of graphics processing power, but in contrast, the latecomer to this generation of consoles couldn't even push an HD image. Of course, the Wii sold well on the basis of its first-party games and novel control scheme, but Nintendo has said its successor would be a much more powerful machine. A developer working on a Wii U game gave some clues that it would indeed have more power than the Xbox 360, but it still isn't enough to handle everything the developer wanted to throw at it.
The source requested that he remain anonymous because he didn't want to be accused of breaking the strict Non-Disclosure Agreements that Nintendo imposes on third-party developers. That being said, he went ahead and broke his legally binding contract by telling a reporter from Develop that the Wii U was about twice as powerful as an Xbox 360. However, that isn't enough for some of what developers want the console to do.
"I've heard [project designers] complain it's underpowered compared to what Nintendo announced, resulting in people having to de-scale their plans," the source added.
This quote is just heresay and rumor from an unconfirmed source, so you should take it with a hefty silo of salt. Until the specs of each console are confirmed by the manufacturers, we will not know what hardware is inside them.
For example, a separate source came forward yesterday concerning the development of the next Microsoft console. That guy told IGN the Xbox 720 would be "six times as powerful" as the 360, which would be "20 per cent greater performance than Wii U."
My math skills aren't that good, but let's say the Xbox 360 has a power of 100. The Wii U would be 200, based on one source, while the Xbox 720 would be 600. That's a far cry from "20 percent" better than the Wii U supposedly is.
So yeah, this is all bunk with maybe a small kernel of truth hidden in there. For my money, I bet that the Wii U is pretty powerful, but the complaints of it not being powerful enough are also probably true.