@Skai Cyan: Not to come off like a total skeptic, but I researched your information about Darkside 2 because that time window sounded very unrealistic. Come to find out, the Wii U port was actually under development for at least six months prior to release judging by the post date of the linked publication--and likely more.  Two weeks isn't completely unfeasible but unless you want a broken product, it would take millions in financial assets and a very large team to pull it off in that short of time: http://www.eurogamer.net/articles/2012-05-23-wii-u-darksiders-2-graphics-at-least-as-good-as-ps3-xbox-360-versions

(As an aside, to clarify for others, the PC supports both DirectX and OpenGL, not just DirectX. Very soon, DirectX 12 on PC will offer a huge performance boost over DirectX 11 allowing for much faster and efficient operation as it operates very close to the metal, or it interacts much more directly with the actual graphics hardware. Performance boosts on the order of 50% to 100% better in real-world testing which have performed by well-respected outside media sources such as AnandTech and Tom's Hardware have done as much to confirm this.)

Now, I agree that Wii U can be somewhat passable but it requires a extraordinary amount of optimizing reads and writes at a ridiculously meticulous level, down to predicting what the player will do next every frame because of the CPU bottleneck. The clear problem is the Wii U's CPU is so slow that it has less processing throughput than that of the Xbox 360's CPU. Ignore the rambling online about architectural differences about out-of-order and in-order processing, SIMD and such between the Wii U and Xbox 360. At the end of the day, the CPU was and continues to be the biggest barrier for developers who want to get decent graphics, as Digital Foundaries, a leading worldwide fabricator of CPU silicon, points out:
http://www.eurogamer.net/articles/digitalfoundry-black-ops-2-wii-u-face-off

To clarify, by referring to general processing here, that is to say that the CPU gets and sends data back and forth from the disc, the memory and the graphics processor and essentially the faster the CPU, the faster you can send perform this cycle of instructions or tasks. The CPU also has to juggle this data retrieval and transmission between other needed common tasks like sound processing, character logic, controller input, online data updates and background operating system management.

Most of the time, a properly balanced system, be it a PC or video game system, will be set up such that the CPU is faster than the GPU can take in the raw graphics instructions for processing, thus leaving the CPU with enough left over to do everything else without breaking a sweat. Not so for the Wii U. Although its GPU is much much more powerful than the Xbox 360 or PS3, without very case specific optimization, Wii U's CPU is not able to pull graphics data in and out fast enough, leaving little for much else.

For games with simple 3D cartoon characters, the Wii U's GPU could have been perfect, but Nintendo would have done well to have put more into the CPU and less into the GPU instead of playing a hardware specs game with everyone. After all, no one cared anyway because the Wii U's theoretical graphics throughput was already a 1/4 of that of the Xbox One and a 1/6 of that of the PS4.

In practice, the Wii U's GPU is much, much less because its CPU doesn't even hold a candle to the Xbox 360's CPU. This is a classic case of poor engineering. Essentially, all of the Wii U's extra graphics processing that is never used is an over-glorified hood ornament. If anything, Nintendo should have sourced a lower spec GPU from AMD and could have saved themselves a lot of money in the long run.

As a computer engineering student with an emphasis in hardware, I live and breathe design. I have heard from other colleagues who also deal with hardware development that some clever compression techniques and specially timed data reads and writes can treat the Wii U's symptoms but none of them can fully fix the issues. The CPU is simply too slow. For that reason, even when the best efforts are made to optimize to the Wii U, something has to suffer and ultimately that is the graphics performance.

The fact of the matter is the Wii U's GPU is hardly--likely never--used to its full potential because  the general processing and graphics processing are not balanced to each other. The Wii U is a classic case of tying a race horse to a mule cart, and no amount of programming pixie dust in the world can make up for that in such a latency (or time) critical application as graphics processing.