The problem with changing light is simply the change in appearence of features - imagine a bump on the plaster, it will cast a shadow in a certain direction. So in one image the black shadow will be e.g. top left, on the next bottom right, on the last no shadow at all. Ergo RC will be confused. If it stays within a certain limit, it might still work well enough, depending on your needs for accuracy. Just try it out for yourself, there is no way around it! 
I personally have good experience with a flash on the camera and no diffusor at all. If it is only a plain surface, there shouldn’t be much of a problem if your lens does not throw a shadow. The vignetting can be counteracted somewhat in RAW developement. If you can’t avoid shadows, make sure you have enough images of each area so the shadowy bits will be outnumbered by images where the area is properly lit.