Easier to rinse. Easier to replace friction media. Easier to visually inspect.
May need a lock-out of some type to back trailer. Possibility of warping if dunked while hot. Tend to run hotter than drum brakes.
For pretty good information, try http://www.championtrailers.com/
If you dunk in salt water, make sure to get either coated or stainless rotors (stainless has something of a reputation for warping.) Ideally, your calipers would be something other than mild steel, as well.
OTOH, I've got drums on my trailer. No problems, yet...