Coming from TEV and complex memory system of the Wii to a modern programmable shader GPU and unified RAM must be a huge improvement. But being more powerful also means that the demands on the quality and amount of assets also increases exponentially from what they were doing before which they have struggled with. Also it's no surprise that developers that have spent 7 years with the X360/PS3 (the early learning curve may have been steep but that was a long time ago) and Windows development would have issues with the immature tools and esoteric platform quirks of the Wii U.
The PS4 and Xbone are said to be far easier than any previous generation thanks to their standard "off the shelf" architecture and great tools (Sony especially has apparently invested heavily into it's dev tools, which is a nice change from the sending technical manuals in Japanese the size of a large phone book and saying good luck of the PS3). But again the increased power will of course mean that the standard of assets will increase which will cause many developers to increase manpower to compensate.
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