For some reason you believe the QBR's assessment methods are 'subjective' in some sort of sneaky, duplicitous, back alley sort of way. Or at least that's the vibe I get.
I do not share that opinion. Strictly speaking any metric that requires evaluation and assessment is going have some degree of subjectivity. But it can be controlled for and mitigated to avoid capricious and arbitrary results. For example, I don't know MartinC except within the confines of this forum.
But, if he and I were to 'grade' not give an opinion but merely to grade/assign credit/fault I'm certain they would be only marginally different. Now if 2 strangers without any standardization can assess something and see the same thing you don't think a group of trained analysts with the aid of computer video, following a strict rubric created by smart football minds could create a useful and football metric?
I applaud the QBR for attempting to assess each pass and assign credit to the individual players for the outcome. It is precisley that effort that makes me respect the QBR. Are there flaws? Sure.
When I look at QBR I mainly care about the Pass EPA which ranks the QBs: Brady, Ben, Rodgers, Ryan, Brees, Eli, Griffin, Peyton, Vick, Romo. And I would say that's about right.