It's the best we got. No one else comes close to this level of detail beyond the stats. Of course they will get things wrong. They don't know assignments, but over a large enough sample, if a guy is getting his butt kicked it will show.
Now don't go around claiming this 70 guy is clearly better than this 65 guy. Way too much noise for that. I also think some grades are fundamentally flawed so must be taken into perspective. Like a #1 shadow man corner getting an average season grade is actually awesome because the daily competition he faces.
Calling it complete trash is just as bad as holding it as gospel. It's a tool that must be understood and used correctly.