Two imperfect techniques
Per-unit calibration works by attempting to measure the color uniformity of each individual camera and store the information so as to be able to compensate for color shading. This method often fails with many indoor non-incandescent illuminants and mixed lighting situations. At best, it is only partially successful; at worst, it may even make the perceived results worse.
Using a blue glass or hybrid filter can improve results, but involves the additional bulk and cost of an extra component. What’s more, the additional filter cuts down on some of the wanted light reaching the sensor, leading to increased noise under typical low-light conditions - again, something that even non-expert users are particularly sensitive to.
More information about color shading