Name me 1 example of how you think it fails to make a valid observation...

I suggest you read this: https://www.profootballfocus.com/about/grading/

Focus on this section:
"How subjective is the Grading?

Many people say that as soon as you start grading, you bring subjectivity into your work. Obviously, to some degree, that’s true.

However, there’s also subjectivity around whether a play was a QB run for negative yardage or a sack, if an assist on a tackle should be awarded and if a catch was dropped or not. Sure, you can come up with a set of rules to determine which is which, but in the end, at the borderline between one and the other, it’s always subjective. It comes down to a judgment call.

The real trick of grading is to define a clear set of rules, encompassing each type of play. If your rules are thorough and precise enough, the answers just fall out. It becomes as easy as determining the dropped pass that hit the TE right between the numbers.

Statistics in their raw form are considered objective. But in our opinion, with the small number of NFL games played each season, raw stats are very often unintelligent. If a QB throws three interceptions in a game but one came from a dropped pass, another from a WR running a poor route and a third on a Hail Mary at the end of the half, it skews his stats by far too great an amount to be useful. Our grading allows us to bring some intelligence to the raw numbers and with many different sets of eyes getting a look at each game, we’re able to avoid the individual biases that may arise if only one person was responsible for grading the game."