The pack contains three difficulty levels to coarsely control the degree of recipe and behavior changes; the mod list for all versions is identical, but the configs and Minetweaker scripts differ.
In all cases, 'difficulty' refers not to "survival difficulty", like danger from mobs or environmental hazards, but to "progression difficulty", in the complexity and interconnectedness of mods and their gameplay. Put another way, all versions are equally easy to survive in, but the harder versions are more involved and more conceptually difficult to advance in.

"Hard" difficulty has the full suite of changes, and is thus closest to the intended gameplay experience. Contrary to the name, it is NOT hardcore, nor unusually difficult in terms of survival or "gamer skill". The "hard" more refers to the fact that progression through the mods is more heavily enforced, a greater requirement of industrial/magic infrastructure, and the fact that shortcuts are rare to absent.

"Medium" is similar to Hard but slightly toned-down; a few of the harshest changes are not present, and the gameplay is slightly more amenable to shortcuts.

"Easy" differs significantly from the design experience, but is the most casual and "kitchen-sink" style of the versions. There are still a great many tweaks, but progression in various mods is not significantly changed from default.

As stated above, Hard is the recommended mode, except perhaps for those unfamiliar with ChromatiCraft or RotaryCraft. Additionally, only Hard contains all the changes listed under "Modpack Changes"; Medium and Easy may not have some or most.