The way the 5e rules are written without facing, you can sneak attack as long as the ally is within 5ft or 1.5m of the target and/or if you have advantage.
Flanking rules became optional apparently so you don't' have to attack from behind like most of us have been taught over the years of gaming. So if I blind the target, crouch and hide in front of the target, shove the target down to prone, or use some sort sort of CC like hold person, or even cast faerie fire or true strike, I can then sneak attack from any position (I'm sure I got something wrong there as I played table top d&d 3 times ever in my life, so correct me if needed).
If flanking rules are permitted by the DM (as they are here), you can gain advantage from side and rear flanks and perform sneak attacks without additional actions.
Many on the forums seem to object because it is too easy to flank and reach the back due to turn based combat and claim improbability or immersion breaking. As mentioned above, bad kind of a bad example, dark souls allows you to square off face to face and in real time you can still manage to get behind someone.
In real life fighting/sparing, I know for a fact it is easy to get behind an opponent in a face to face fight with proper footwork (if they go forward you move in an oblique and angle past them). Add in manipulation of the limbs and it is even easier to get behind someone in a fight...so I guess grapple as some have mentioned is a missing skill in this game.
So "back stabbing" sneak attacking is wholly possible from the front or in a 1 on 1 fight, even with d&d rules.
Anyway, why are so many of you worried about immersion and reality in a table top game revolving around magic and dragons, but it is immersion breaking to you that a guy can stab someone in the back at will?
