Friendly fire damage is significantly increased where a stray shotgun blast or few bullets can incap a Survivor. Realism mode removes the colored auras around allies and items, making communication about who and what is where a must. People who don't know how to pull this off might as well map quickload and quicksave to a mouse button.
Now, picture this in Elite, where, in one or two shots, you die. The threequel, although easier than its predecessors, still has a harder-than-hard difficulty in the form of Insane mode, which is unlocked by completing the game on a lower setting.
They're not kidding. Checkpoints are drastically reduced. On Hardened, the game becomes a real challenge.