It is not only about intensity.. Different colors have different wavelengths and hit the sensors in different places, not only at one spot. So there might be color problems because of this.
But mostly means some colours will not hit the sensor at all, or only with very little intensity. This is pretty much the same thing as very bad vignetting, when there is no intensity at all. It can't be easily corrected, because you have no information at the first place.
Also, correcting intensity will always create noise, so you cannot correct it too much or the image quality will decrease obviously.