Rear Disc Brakes are better than Drum Brakes. Period.
Why?
Drum brakes takes a long time to dissipate heat. When mud or sludge/debris goes in, you will need to disassemble it to cleat and take out the dirt that lessens your braking force.
Disc Brakes are very efficient in cooling as the rotors are exposed in the air. Muds and off-road debris just gets wiped off as you press on the pedals.
so when you are building an off-road /adventure rig, this is one upgrade, you have to consider.