I've got discs on my double axle trailer. They get dunked once a month. Just did a complete stripdown and rebuild for the journey to Oban.
They are definately much better than drums IMO. The best bit is that you do not have to expose the bearing to work on the brakes. The problem is that the calipers cannot be completely dismantled and the auto reverse mechanism has a tendancy to stick. Club hammer seems to sort it.
Complete strip, grease (as best you can) and rebuild every 2 years seems to keep them working. Pads seem to be lasting. You can give them a good flush with a power hose from the opposite side of the boat.
Cable load adjusters all stuck this year. Replaced with polished stainless plates and stainless forks.
The belden cables are plastic coated and still run free.
Discs are lasting better than my pickup.
What we need are these:
Completely stainless less hydraulic brakes. Nice.