If they want wii u compatibility it will be designed with that in mind even if using different architecture. They will optimise a few things to enable full speed wii u emulation. As previously stated the wii u gpu its pretty standard although low power radeon hardware and the cpu is absolutely dire so no problems with that. The memory bandwidth is very low in wii u except for the 32MB of high speed memory. That 32MB could be difficult to emulate but the NX may well have a similar memory arrangement.
Full wii u compatibility is no problem at all and wii emulation could be done easily fully in software I'm sure without having to worry about optimising anything.
I think people have to remember with Microsoft's approach that they use patches to bypass emulation issues. So its only partial emulation and part new code. It's a different approach and the xbox one probably isn't powerful enough to emulate the 360 fully in software. However the 360 in many ways is more powerful than wii u and the xbox one is likely less powerful than NX at least in cpu terms so its an easier job.
So looking forward to NX hoping its a return to decent Nintendo hardware that will unleash the full potential of Nintendo's game developers and not having to fight the hardware all the time.
I would truly love it if the NX had a top loading optical drive and ran wii u, wii and gamecube software but the reality is I think no optical drive so wii u compatibility is only going to be for digital downloads so I'll need to keep my wii u anyway as I have few digital games.