In describing over/under moist, it's important to make a distinction between the growing medium and the soil.
It is very easy for the growing medium to become over moist, but very hard for the ambient air to be over moist.
If the relative humidity in the air surrounding your cuttings is less than 75%, then leaves on a new cutting will start to curl up and die, regardless of the moisture level of the soil.
If the soil is too moist, the cuttings can start to rot, which will attract fungus gnats, which will in turn lead to more rotting, and you will likely lose some percentage of your cuttings.
It is quite possible to have air too dry and growing medium too moist at the same time.
To address moisture level of the soil, best solution is to start with growing medium that is at the right moisture level (moist to the touch, but you can't squeeze water out of it), and water very infrequently, and not much at a time
To address moisture level of the air, you need a partially closed container. This can be a large bin, as in the picture, or it can just be an upside down pot on top of each cutting. You don't want it fully enclosed, or the stagnant air might encourage mold and decay. But if you allow in too much dry winter air, you will dry out the new cutting growth, until the roots are robust enough to support it. The degree to which dried out early top growth hampers the ultimate survival prospects of a cutting is a topic I haven't seen discussed or considered. I think most people (including myself) assume it is a bad thing, but I haven't seen any real evidence or studies to confirm this.