One of the best games of all time is Half-Life. That game had little realism whatsoever as a MIT scientist managed to single handly kill countless soldiers, aliens, and vehicles with a crowbar and various guns. And don't get me started with Half-Life 2.
Well I imagine blindsiding someone with the butt of a rifle at full force would hurt them pretty badly let alone doing it twice.
Also slamming down on them with a pistol, or literally anything really..., twice hurts as well.
I mean you can also complain about melee is unrealistic because I can jump off a 20 ft tower and live but get pulled off a 1 ft ledge to my death.
I never really saw melee as a problem outside of lag. I mean most people who go in for melee, also shoot at you, so if it even does become a 3HK, chances are it won't change much since gunfire will easily make up for the damage reduction with 1-2 bullets more.
I just want Tenzin to take out his Kukri and chop off Marlowe's head.