@fibs well I played with extra dmg from back and rear attacks in Age of Conan for years and never encountered the problem you described. And the developper Funcom is not know for making flawless games, boy did Age of Conan have lots of problems. So maybe the problem is just ib Dark Soul and the other game you described.
And ofc it matters what direction for character is facing in a game with isometric view. In Albion Online on my tank I jumped toward my enemies, aiming with my mouse on wich side of my enemies I would land, so after landing I would aim with my club in what direction i want to use my 20 meter knockback, to knock them into a group of elite mobs.
I don't know why the remark about character turn time is directed at me though. I didn't brought it up and if it is upto me the turn time is as fast as you can click with your mouse.