I think the term Remote DSB is a bit misleading. According to Dr Ron, the surface area exposed to the water column needs to be a certain size in order to facilitate the DSB process as a whole. Ive read quite a bit on those when they first hit the scene a few years back and I am convinced they are not true DSB devices, but nitrate/detritus sinks. This view is based on the large number of people who attempted DSBs in their smaller aquariums many years back, only to have them 'top' out or stop functioning in a short bit of time. In this particular case, that is made less important because you can easily replace the bucket of sand, unlike breaking down a tank to replace a sandbed.
Does that make them useless? Not at all, because they do appear effective for a time. Just view them as large sand filters with a bit of nitrate reduction capability and not a true DSB itself and that will help. Remember the term DSB is really for beds in a tank that are sustainable over the life of the tank.
That said, 3/4 full is about what I have seen as effective. The risks are in the pumps, filling and draining the bucket takes some fine tuning and adjustment and is a failure point. Other than that and the space, they won't hurt anything and the depth doesn't matter once you reach 6 or so inches. Once you work out a safe fill/drain mechanism you should be good to go.
Just remember to still keep an eye on nitrates after you put it in, you'll see a decline over time, but unless you replace it, the number will start to creep up after some time has passed. When that happens, swap it out, re-seed and reconnect.
