The seats are narrow, so the elastomer seats tend to deform into the shape of the ball.
Trunnion valves are more expensive to make than floating ball valves. They require less torque than floating ball valves because the differential pressure forces are absorbed by the trunnions and not by the seat. Depending on service conditions and size, Trunnion Mounted Ball valves become the most economical choice for valves over 10 to 12”
Trunnion valves are available up to 2500# class, and are only limited by temperature.