Literally every NFL team is their client lol.
Their grades are certainly a black box to the outside and I get the confusion because their grading aren't just old-school box stats that everyone can see (they lean toward being process-oriented over results-oriented) but their team clients get to see it all. Also partnered with dozens of CFB teams.
Just as example: Carvin could allow a pressure or whiff on a block, but HH escaped a sack on the play by a hair. That doesn't show up on the box stats, but it happened. On the reverse, an OL could block their man for 5 seconds until the QB runs right into his defender for a "sack" that looks bad on paper, but wasn't his fault.
They certainly have their downside and is no way perfect, for instance they can't always know (though can estimate) every player's assignment on every play. But no external service will know that either. But I get the dislike, it's tough to accept something you can't see on paper and feels like a black box.
But it would be closer to if you went back and watched the player, on every play, for the entire game. Did he fill his gap that play? Did he take up 2 blockers? Did he cover his WR throughout the play that led to a coverage sack/throwaway? There's just so much more to football than box stats.