to be effective when sailing the club 88 style friction brake to the rear wheel or wheels is about the best for a gravelly or hard surface, the stick brake(LLM plans) is great on salt and sand or mud. for DRYmud/clay I would go the wheel brake.

brakes on the front , as Gizmo said have a reduced effectivness due to the low weight,, and of course remove the steering option, I think the ones Ive seen on fives are just for parking up. i have seen a few M/C and scooter wheels used and the internal drum brakes employed, the owners reckon they are brilliant, but personally I reckon its just too much weight to be putting there

those fancy discs on the blokarts come with a warning not to use them on sand.
dont forget you should always stop pointing into the wind so pushing the boom out and aerobraking is another option