I love using cartons with duck eggs, and get better hatch rates than before (although there are other factors involved too, so there may not be a causal relationship there).
What I like: the ducklings can't push the unhatched eggs all over & cause them problems; it takes longer for the ducks to get out of the egg after popping off the top, which means they build more strength and coordination so they are less likely to injure themselves; I can position all the eggs where I can see where they will pip; it keeps everything orderly in the incubator. I get 80-95% hatches, so I'm pretty sure it's not messing me up too badly.
What I don't like: every once in a while, a duckling will fall asleep on top of a section of eggs that are trying to zip. This can cause the zipping eggs problems, but it's unusual because there's really only about a one-hour window during which it can cause them grief for there to be weight on top, and the weight has to stay there for an hour or more, which is unusual. But I have had it happen, and I ended up having to help all four of those zippers out of their shells--with no long-lasting ill effects, I might add.
Humidity really isn't an issue. I use styrofoam usually because it's easier to make sure it's really clean. When I use cardboard, it actually tends to raise the humidity because if it comes in contact with water, it wicks it up and then releases it much faster (remember that it's surface area, not volume, that raises humidity), so I end up with really high humidity. But I don't like the moisture being held right against the eggs by the cardboard, so I use styrofoam most often.