Number of voxels controls the ability of the renderer to represent detail in the cloud (that derives from the Density Shader). In other words if you have a very detailed/multi-octave Density Shader, but low number of Voxels, you will not get as detailed a cloud as you might expect. So higher voxel count = more detail, especially in cloud "wisps" and in internal cloud scattering/lighting. I would generally suggest leaving it at default while setting up a cloud, and then if you feel you have good settings for a detailed cloud shape, but you are not seeing enough detail, either in cloud edges, or in shadow/interior scattering, then try increasing Voxels and see if it helps. Don't assume that higher voxel count is always a lot better though, as it increases render time and especially memory use.
Cloud Shader v3 is a generalized new model for more realistic light scattering in clouds. Easy Cloud is just *one* (easier to use/simplified) node that uses the v3 cloud shading technology. But v3 clouds can be just as controllable as v2 clouds using the "Generic" preset, as you found.
- Oshyan