Simpler, easy to maintain and more reliable then Merlin. Garra will start maybe with Merlin 66 who give you about 1,720 hp, Centaurus IV ( the same as Tempest ) give you 2,520 hp. Thats a lot more power even if radial is heavier and less aerodynamic.
The radial has more HP to compensate for the added weight and lack of aerodynamics. So it ends up being about the same worth as the Merlin.