He's a perfectly capable 4th guard with no weaknesses, but no strengths either. He is an ok shooter, ok defender, ok passer, ok driver. But you can't rely on him to run the second unit, or to shut down the other team's best creator or their second unit's best creator.
He is the tofu of NBA basketball players. Perfectly capable of playing a role, but is not going to add anything special.
Rivers is probably a better defender than Rozier. He also may be a better team offense guy than Rozier (although Rozier is a better creator). But I'd rather have the possibility of Rozier catching fire in the 3rd and hitting 5 threes.