Short answer: Composite rates can be higher than age rating for a young workforce, and they can shift at renewal as the group’s makeup changes, so the simplicity comes with less precision.
Composite rating’s blended approach has trade-offs. A group with many younger employees may pay more than it would under member-level age rating, because young workers subsidize the blended rate. Composite rates can also move at renewal as the group’s age and tier mix changes, even if no one’s individual circumstances changed. And not every carrier or state offers composite rating for small groups. It’s a convenience-versus-precision decision.