With regard to the lack of standards and objective metrics for comparing soaps, I agree that there are too many variables at play for scientifically accurate comparisons. Once fragrance preferences and lather quality predilections are added into the equation, all bets are off on whether you would agree with any given reviewers opinion.
One of the best attempts at doing comparison reviews are some of the videos done by ShaveBusta in which he lathers two soaps with the same types of brushes, for the same amount of time, with the same water, with the same amount of beard growth and shaves each side of his face, with the same razor, with each of the two soaps being compared. He then immediately gives his impression of the shave for each side of his face. I think that's about as close as one can get to being able to make an accurate soap comparison.
One of the best attempts at doing comparison reviews are some of the videos done by ShaveBusta in which he lathers two soaps with the same types of brushes, for the same amount of time, with the same water, with the same amount of beard growth and shaves each side of his face, with the same razor, with each of the two soaps being compared. He then immediately gives his impression of the shave for each side of his face. I think that's about as close as one can get to being able to make an accurate soap comparison.