Hi Paul,
We have electric drums on our current camper, but have used other systems on other trailers.
Our TrakShak camper had cable override drums, and they worked well when adjusted properly.
They need to be run with plenty of slack in the cable, or they will be "off-on-off" as the trailer suspension goes up and down (leaf springs). A good, basic, low maintenance option. Very little to fail in the middle of nowhere.
I haven't used cable override discs, but have heard they are not ideal for off road trailers. Apparently, the corrugations knock the calipers about. OK for on road use, though.
Our 8'x5' tandem box trailer has hydraulic override drums, and they also work well, with on road use only. They've been on the trailer since I built it over 16 years ago, and haven't been touched. I expect I will need to replace the wheel cylinders and master cylinder some time soon, but they've been fine so far. I have a mate that has hydraulic overrides on his Pioneer camper, and he is in the process of changing to electric. He's had a few instances of rock damage to the steel hydraulic tube at the wheel cylinder. If you are not using for serious off road work, this may not be an issue for you.
Electric brakes are very good, but they are not maintenance free. The magnets will need replacing every now and then, and spares should be carried on big trips. You need off road magnets for off road use, and even then they need to be inspected regularly.........but this can be done at bearing maintenance time.
If it were me, my first choice would be electric drums..........with cable actuated drums second, then hydraulic drums.
As mentioned by corynmelnkids, if using override brakes, make sure the coupling is rated for your setup.
Cheers,
Geoff