Phoenix Point doesn’t really have difficulty settings that alter the core gameplay. You fight the same AI and against the same enemies with the same hit points and the same armour on the same maps on all difficulty settings. Rookie or Legend, the same.
On Legend, you might get some more enemies on some maps—especially story maps. Or pretty much only story missions.
The main thing the difficulty settings change in Phoenix Point determines how quickly the Pandorans evolve (fast on Rookie and stupid-fast on Legend), how many skill points your soldiers earn per mission (12 on Rookie and 5 on Legend), the starting stats for your soldiers, the number of soldiers you start with, the cost to recruit new soldiers (double the food cost and no gear for any soldiers on Legend), and the amount of resources you start with at the beginning of the game (fair in Rookie and totally gimped on Legend).
All that stuff impacts the early game. So, in other words, the difficulty setting determine how long it will take you to get out of the early game and get your bases and squads up and running.
As soon as you’re in the mid-game, well, there is pretty much no difference between difficulty settings. All enemies eventually evolve to their ultimate forms; it just takes twice as long for that to happen on Rookie.
But once the Pandorans are fully evoked and the player has squads of dual-classed, level-seven soldiers, it’s the exact same game on any difficulty setting. Rookie or Legend, pretty much the exact same difficulty from the mid-game through to the final mission.
And any of these “Legend only” players exploiting the Raid mechanic is not only cheesing the game but completely breaking the difficulty settings. In the current version of the game, Raiding havens is like typing in a cheat code to get free resources with the key differences being that typing in a cheat code would take less time, whereas playing the raid also gets you a ton of dropped loot like weapons, ammo, and med kits—in addition to the thousands of resources hauled in with each raid.
“Legend only” players. LOL
In fact, until that exploit is patched out, the difficulty settings you choose at the beginning of a new campaign are merely flavour-text.