From what I understand 999dice is not a scam
But by scam do you mean not fair or they dont pay out?
999dice is a scam.
They pay out fine, the problem is in the fairness of their game. They hide their fairness of the game so well that no one notices, but in reality they can rig your rolls.
You can read the thread here:
https://bitcointalk.org/index.php?topic=948965