But, at the same time, at black belt and higher, there should be some commonality that can be compared. You can compare two masters degrees, even if they're not in the same academic discipline. You should be able to roughly compare two black belts, and have an idea that both are dan-ranked.
I wouldn't even grant that. A Masters in Criminology is a Masters, a terminal degree. A Masters in Electrical Engineering or Studio Art is a terminal degree in a completely different field. You can't compare the particular skills and knowledge of one to the skills and knowledge of the other although you can say something about the overall amount of time and preparation the other guys have done. A dissertation on the theory of Deviant Psychology and social reintegration contrasted between South Korean and Texan penal systems is great. But it doesn't deserve any standing in Microwave Circuit Design and vice versa. Neither of them has anything to do with sculpture or printmaking.
A Karate board that judges your skills in Kendo and awards rank in Karate based on them has taken leave of its integrity. If it awards you Kendo rank it has also taken leave of its senses.
When I talk about styles awarding rank it's a matter of background. I started off in Judo. Every test was done in front of a board of examiners with at least a certain rank in Judo. The certificate came from the USJA (or was it USJF?). It was the same when I tested for rank in other systems. There wasn't just one examiner. A group of them had to pass on everyone, and the organizations of which they were a part signed off.