I think that rather than comparing specs, you must look at it as when these systems come to market in relation to one another, and how they compete with one another. The Wii was released alongside the PS3, and both systems built up an install base alongside one another, despite there being no question that PS3 was more technically capable of displaying more photo-realistic images.
At the same time, surely no one would reasonably argue that 3DS and PS Vita are not competition for one another. They are both proprietary gaming devices released near one another and they have similar functions. They are indeed both eighth generation handhelds. How else would you speak of them? Surely 3DS wasn't written off in 2010 as "not a next-gen handheld" simply because PS Vita is unmistakably more powerful.
Therefore, if we can reasonably agree that Wii was in fact in competition with PS3 and 360, and was on the market during the seventh generation of consoles, just like the PS3 and 360, then we can also reasonably agree that Wii U, since it exists and is selling alongside the X1 and PS4, is indeed an eighth generation console, same as the other two. Making an arbitrary distinction like "it's graphics aren't as good so therefore it doesn't count" is complete nonsense. Because, if that's true, then 3DS doesn't count as an eighth gen handheld, and Wii didn't count as a seventh generation console. And how can that be? They were there. They sold. They sold better than the competition. 3DS is steam-rolling it's competition. It proves what many people refuse to hear: More horsepower does not equate to more success in the market. In fact, the opposite is true: Usually, the least-powerful (more affordable, larger library of games) system ends up being the one with the largest install base. Why people still ignore this fact is totally lost on me when I see threads like this.