Past that and in my personal opinion, the stuff in that 1-3 OR 8-10 score range plays (call it deal breakers) SHOULD be weighted more.
I think it both is and isn't. I also don't think they claim to be that.
Down and distance is taken into account, from what I can tell. Giving up 9 yards on 4th and 10 is a win, because they got the stop, for example.
Making a more routine play on 3rd and 6 is, I understand it, rated the same regardless of the yard marker. It's one down in either case. Yes or No, did the player do their job? Yes, this is subjective, as we don't 100% know the playcall, but other teams do this against their upcoming opponents. It's good enough as long as you understand what their are trying to measure.
For example, they've dinged Rodgers in the past when he's drawn the defense offsides. He is always willing to make riskier throws on those free plays, so he would get a wag of the finger, regardless of outcome, because it was a dumb throw. Their grading excludes the free-play-ness of the decision, and that is also fine, as long as people remember that. (Also could be considered to be the right way to do this. The 2014 playoff loss against Seattle? Rodgers threw a pick to Sherman early? He drew the offense offsides but the ref didn't throw the flag, so no free play.)