It's difficult to see how the wii u is that difficult to develop for when it's using the same cpu architecture as wii and the gpu is an old radeon design somewhere between ps3/360 level and ps4/xbox one level. Surely well documented.
Darksiders II has missing graphic detail on wii u and is inferior to 360/PS3 as are most multiformat games even those released recently.
The development difficulty seems to be getting the wii u to perform to 360/PS3 levels and they aren't managing it.
It's pointless viewing the wii u as a next gen performance console. Both 360 and PS3 will get more cutdown versions of ps4/xbone games and the few that are developed for wii u will probably run better on ps3/360 anyway.
You don't need to read this article to know the wii u is technically weak you only have to look at the games and its construction using low spec parts and fabrication process. It draws hardly any power despite using a power hungry fabrication process.
Nintendo had its eyes on manufacturing a very low console and selling it at a huge profit. Last time it worked for Nintendo but this time it failed and now they risk losing all the profits they made with wii by having to support and develop for a console that has a small userbase.
They should have done a quick fix like the 3DS dropped the price and done an ambassador program for early wii u buyers early last year. Instead they held their nerve and have paid the price for it. Now they will probably have to drop the wii u price even more dramatically to get reasonable sales figures.
You cannot expect people to pay top dollar for a low performance console with a limited range of games available.