CPUs in Wii and Gamecube (tossed both of mine out today btw, since I don't have any gamecube games anymore except for Metroid Primes 1/2, and wuu runs wii games (including MP Trilogy) flawlessly and in 640P) most likely only did geometry for skinned characters; not sure if it actually did any lighting at all. The GPU had pretty solid fixed-function T&L stuff, except for aforementioned lack of skinning/geometry shader support.
Oh, I'd expect rendering to be completely different in every single way for a wuu title compared to previous nintendo games. Feature-wise, the wuu is about four GPU generations ahead of wii, and the one thing they both have in common is they both rasterize textured polygons, and that's it...
Of course, how wonky wuugpu may be under the hood depends on how much of the wii backwards compatibility is 'visible' when the thing is running in wuu mode. It may be that functionally it's pretty much a standard radeon 4k series with eDRAM tacked on when running in wuu mode (and devs probably don't bang the hardware directly anyway but rather access some OGL variant.)