You have to realize that NX is going to be next gen compared to ps4 and xbox one. Wii U is weird in that in most ways it is next gen compared to ps3 and xbox360 except the processor speed. IT has more ram and a superior GPU, which would mean it should be way better than those tow, but according to developers its CPU is weird and not as fast in clockrate. Developers could take advantage of the CPU in different ways, but they would have to reprogram the games.
As a programmer I understand some of this. When I write my code I test it constantly. There are some types of code you could write that's inefficient and may not be noticed on a faster cpu. On a slower CPU you might be like WTF? IT costs too much money to have someone rewrite the code just to get it running on Wii U. There is a lot of slowdown on Watchdogs, this is because what code works on ps3 and xbox 360 does not work on Wii U's cpu. I can imagine that they might have to change some gameplay to get the game running smoothly on wii U, but they didn't.