Stealth mechanic is going to be whatever the 5e edition rules states it's going to be, so I assume you can magically stealth behind somebody during combat and make a sneak attack? As long as that opponent doesn't see or hear you?
Sneak attack is a precision strike to an unprotected and vulnerable point in the enemy´s body. You do not need to be stealthy for that, but it helps. In fact stealth is a totally different mechanic unrelated to SA. Basically stealth in d&d is a skill check like all the others.
Basically as long as the enemy is not inmune to sneak attacks because they do not have vulnerable points(ex: undead, elementals...) and it is not aware or targeting the rogue directly, you have a sneak attack on them. Of course you can have it if you are invisible (also have bonuses) but it is not needed anymore: You can attain it if you have advantage, or the enemy is blinded, if you used skills like surprise attack or feint(or their counterparts in 5e), or if the enemy is attacking another party member. You do not even need to be behind the enemy or have melee weapons anymore (as long as you are closer than 2m).
Ed: What I would like to see in BG3 is a mix between the tabletop and a videogame: You need the skill to be stealthy, but the success not only depends on your skill and the skill of the enemy, but also in your positioning, equipment, the ilumination, environment characteristics, the use of distraction items or tactics, etc.
