as you mentioned, all the grading is highly subjective but due to them assigning number grades, has been interpreted as objective fact. my biggest issues:

1.) they grade the TV broadcast -- now, they say the watch all-22 later in the week and make adjustments, but it's hard to overcome initial bias. how you grade anybody in pass coverage and most quarterbacks without all-22 tape is beyond me, because you can't. not to the level of specificity they claim. this is actually a near-fatal detriment.

2.) grades swing wildly based on plays with unknown assignments -- the only people that know what the coverage call was are the players and coaches. just because a defensive back looks like he should've rotated over, or looks like he was in man coverage the entire way, doesn't mean there wasn't a bust somewhere else that put him in a bad spot. and if they blame the wrong guy for a 40-yard gain, that puts a huge negative on his grade, possibly for the entire season. same thing goes for quarterbacks who throw to a defender -- was he reading the coverage wrong, or did his receiver run the wrong route?

3.) the mistakes roll downhill -- even their staff admits that a couple bad plays in a few games can turn a decent grade into a negative for the whole year. all this subjectivity is piling on top of each other for better and worse.

I really like the idea of scouting every player for every game, because football is an extremely hard game to quantify (unlike baseball or basketball). but for the past few years the media and fans have been spouting off PFF grades like absolute truths, when they really are as subjective as your or my opinions.