Coverage Sack Rousseau has never been a difference maker. He is an average to above-average D-end in this league.
I personally didn't think he was worth re-signing to a second contract but I understand that Beane didn't want to create another hole on the defense and there is probably a moderate step down going from Rousseau to Epenesa
He's never going to be a guy that takes over games or makes a big impact. He is a decent edge setter that will occasionally get pressure on the QB but will be largely invisible for stretches of games