A bigger impact on weapon choice as I talked about could be part of it. If there are some Jackal snipers on higher difficulties and not on lower, that changes how you approach encounters. Some of this exists in the games via upgrading the ranks of enemies that spawn or have a chance to spawn, but it's inconsistent in how that plays out.

Ideally I don't really think enemy damage or health values like you mention in your doc should be a factor at all. Higher ranking Grunts having more armor or Elites having stronger shields makes sense, but I don't see why that should change from difficulty to difficulty. While it's had some issues in regards to balancing, I think 343 was smart to try and keep campaign and multiplayer feeling the same—you aren't picking up a weapon in campaign that's a hand cannon, only to find it's a peashooter against human opponents. Likewise, why should a plasma bolt deal more damage on Legendary? Make the higher ranking enemies better or more frequent shots, but the amount of damage I deal with their weapons should equal the amount of damage they do with them. That means I know how much of my clip I need to empty into that Minor or Major to bring them down, it's just the surrounding gameplay that leads to more or less challenge.

Mostly I think it's also important to prevent the players from being forced into crutches (i.e. noob combos) or from being forced away from using gameplay elements. In Reach, the whole "melee" part of the equation was removed from me. That Elite will kick me dead before I can reach him every time, and Bungie gave me a lot of DMR ammo, so I guess this game is "shoot at stuff from range slowly and methodically, shoot at the reinforcements, repeat." And that was stupid and boring and frustrating.

Likewise, Halo 2 left you with only the noob combo as a way to beat stages. Things like The Flood and their different resistances mixed things up a bit, but it's never made too much of a difference between "no snipers and no overcharging"; in CE if you didn't have a shotgun and pistol for the last third of the game you were doing something wrong. Halo 5's revamping of the Knights is really the first exceptional enemy in a Halo game since Hunters; if you're a good marksman you can finish a Knight rather trivially with some well-placed shots, but it also means making sure you've got ammo to take the headshot when they scream, or that they're turned the right direction.