John-
Discs work equally well in forward or reverse, and require a back up light activated solenoid or mechanical lock out to back up easily.
Drum brakes are self-energizing the way the shoe is pulled into the drum once activated in forward travel, and you can back up w/o the lockout in the opposite direction. The lack of a self-energizing effect in disc brakes is why they require higher pedal pressures and usually need power assisted brakes in cars and trucks.
Mike's right on about the ease of clean up and flushing of discs. They also dry better, reducing rust.
Discs do drag more, with zero clearance between the "resting" pad and the disc. But you CAN easily see how much brake lining is left with a disc system, whereas the drum has to be removed to see the drum shoes. Some disc drag helps heat up the discs on short drives to dry out the system after flushing. Better to flush out at the top of the ramp and then drive home, drying out the brakes, than flush at home later and then let the system sit and rust!
For salt water use, discs are much better for their ease of flushing and drying out, especially if they are made with stainless rotors and other non-corrosive parts (some parts will be brass or other non-corossive metals as they can be made cheaper (less expensive metals/easier to machine) and are just as serviceable in some parts applications).
I love the electric over hydraulic concept, the only disadvantage being the cost. If I remember correctly, dual axle Kodiak stainless disc brakes with electric over hydraulic brakes and all the goodies, including the break-away battery to power the trailer brakes after a separation, was abut $2500.
C-Bill, who has a CD-22 and lives in Carson City, Nevada, has a very slick set up like we're talking about on his trailer. You might email or PM him for more information.
Joe.