There's another implication of the UNIWB method which seems unhelpful in many situations.
As I understand it, but please correct me if I'm wrong, the UNIWB method allows for a greater exposure, without clipping highlights, than one would usually be able to use.
It's basically like taking a DSLR that has a true base ISO of 100, and giving it a true base ISO of 50. One consequently gets approximately (maybe) a one stop DR and SNR advantage.
Now this is fine if a slow shutter speed is okay. In a sense, it's a method of turning a 35mm DSLR into an MFDB with regard to DR and noise at base ISO.
The problems arise when slow shutter speeds are not appropriate, and one needs to either increase ISO or underexpose.
In these situations, the UNIWB, it seems to me, is of no value.
If you are not sure about this, let's consider a specific example. I need (or want) a shutter speed of 1/200th at F8. The lighting conditions determine that I need to go to ISO 400 to get an ETTR, using Auto WB.
Guillermo's method suggests I can use 1/100th sec exposure at such an aperture and ISO, and consequently achieve lower noise. But 100th sec is too slow. I need to use 1/200th sec.
So, Guillermo's method suggests that I can use 1/200th at ISO 800, without blowing highlights.
Now my question is, why should 1/200th at ISO 800, with UNIWB, be better than 1/200th at ISO 400, using the camera's Auto WB?
I hope no-one is getting a headache reading this.
Or to put it another way, why should 1/200th at ISO 400, with Auto WB and correct ETTR, be worse than 1/200th with UNIWB at ISO 400, underexposed by one stop?