Primary Topic Links:
-> https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=713497&page=1
-> https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=716036&page=1
-> Primary feedback thread, multiple times

Summary: In the 5e ruleset, Advantage/Disadvantage is the most powerful impact on gameplay. This is true both from a mathematical standpoint and from a player/DM perspective. Because it is so powerful, all sources of Adv/Dis in 5e come at either a cost of resources or a potentially penalty to the user, to balance out the sheer power of the mechanic. Currently, BG 3 subverts this balance by providing always available sources of Adv/Dis in the form of Height and Backstab, both of which require nothing more than having your character in the correct location on the map (note: this is exacerbated by the Jump/Disengage/Stealth problem discussed elsewhere, but exists even if that is fixed). This makes the 5e sources of Adv/Dis nearly useless as they are all more costly or penalizing, thus invaliding literally dozens and dozens of class features and spells, completely ruining the balance of entire classes. Removing Height and Backstab based Adv/Dis will go a long way towards making the game far more balanced and play like a D&D game.

Main Discussion:

The Maths: - In 5e, in general, having Advantage is roughly equivalent of having +5 to your roll. Disadvantage is roughly equal to having -5 to your roll. This means if one character is rolling with Advantage, and the other character is rolling with Disadvantage, then there is the rough equivalent of +-10 between their rolls. Additionally, Advantage doubles the chance of rolling a critical hit, and makes critical failures much less likely (5% normally vs 0.25% with Advantage), vice versa for Disadvantage.

Next, one of the core theories for game balance in 5e is called "Bounded Accuracy". This term means that players and NPCs generally have limits to how high they can boost their static modifiers to rolls. There is a 'bound' on just how 'accurate' a player can become. This was a huge shift in D&D when it was introduced. In prior editions of D&D, players could achieve truly insane modifiers to their to-hit, to the point where attack rolls were reaching into the 1d20+100 range, which just creates stupid arms races between monsters and players. By reducing how much a player can add to their to-hit, WotC (the publishers the D&D rules) made smaller bonuses much, MUCH more important. For example, Bless requires both a spell slot, concentration, and is limited to three targets, and only provides an average of +2.5, half of Advantage.

Bounded Accuracy is why Adv/Dis is so impactful on gameplay. There are a few class abilities that can add a higher static modifier (such as a War Cleric's Channel Divinity that can add +10 to one single attack roll) but those are rare and always limited in amount. It is a large reason why 5e is generally much more balanced that prior editions of D&D with far less ways to truly 'break' the game. It also makes the gameplay much smoother because enemies to-hit and AC do not need to increase as much as you get higher level. On page 274 of the DMG, there is even a chart for rough AC numbers based on a creature's CR (Challenge Rating. The higher the CR, the more 'powerful' the creature):

CR 0-3: 13 AC
CR 4: 14 AC
CR 5-7: 15 AC
CR 8-9: 16 AC
CR 10-12: 17 AC
CR 13-16: 18 AC
CR 17+: 19 AC

Look at those numbers. Over the course of 17 'levels' of CR, the enemies AC only increases by an average of +6. Just having Advantage almost cancels that growth out entirely. Here is an analysis of the actual monsters made available from WoTC and their respective change in AC -> https://i.stack.imgur.com/a6rlg.png

Lastly, if you take a level 1 character and a level 17+ character, give them the same stats and the same weapon, the total difference in their to-hit roll will be....+4. That's it. A level 1 character has a proficiency bonus of +2 and a level 17+ character has one of +6. The difference between these otherwise the same characters is less than the difference from Adv/Dis. That is how strong Adv/Dis is mathematically.

Hopefully by now, you can see why getting Adv/Dis is such a huge deal in 5e rules, and why being able to have them should be considered such a huge impact on the mathematics at play.

The Gameplay: - Here is a non-exhaustive list of class features and spells that grant Adv or impose Dis for characters between levels 1-4:

General Actions:
1. Dodge/Help
2. Dropping Prone against Ranged Attacks

Spells:
3. True Strike
4. Vicious Mockery
5. Cause Fear
6. Command (certain instructions)
7. Compelled Duel
8. Ensnaring Strike
9. Entangle
10. Faerie Fire
11. Find Familiar (Help Action)
12. Fog Cloud (Depending on types of sight)
13. Grease
14. Guiding Bolt
15. Protection from Evil/Good (Against certain enemy types)
16. Sleep
17. Snare
18. Tasha's Hideous Laughter
19. Zephyr Strike
20. Blindness/Deafness
21. Blur
22. Darkness (Depending on types of sight)
23. Heat Metal
24. Hold Person
25. Invisibility
26. Maximilian's Earthen Grasp
27. Shadow Blade (Depending on lighting)
28. Web

Class Features:

29. Barbarian - Reckless Attack
30. Barbarian - Wolf Totem
31. Barbarian - Ancestral Protectors
32. Bard - Words of Terror
33. Cleric - Warding Flare
34. Cleric - Invoke Duplicity
35. Druid - Multiple Wildshape forms that grants Pack Tactics (Wolf, etc)
36. Fighter - Distracting Strike
37. Fighter - Feinting Attack
38. Fighter - Goading Attack
39. Fighter - Menacing Attack
40. Fighter - Trip Attack
41. Fighter - Fighting Spirit
42. Monk - Patient Defense
43. Monk - Open Hand Technique (knocked Prone)
44. Paladin - Conquering Presence
45. Paladin - Nature's Wrath
46. Paladin - Abjure Enemy
47. Paladin - Vow of Emnity
48. Paladin - Dreadful Aspect
49. Ranger - Umbral Sight
50. Rogue - Assassinate
51. Rogue - Master of Tactics
52. Rogue - Cunning Action (Stealth)
53. Sorcerer - Eyes of the Dark
54. Sorcerer - Tides of Chaos
55. Warlock - Hexblade's Curse
56. Warlock - Pact of the Chain (Help from Familiar)


56 different spells, actions, and class features (I'm sure I missed some as well). Every single one of these costs a resource or imposes a penalty for using. For example, the Dodge Action takes your characters Action for that turn. The Barbarian's Reckless Attack makes the Barbarian grant Advantage to anyone attacking it for a turn, making it significantly more likely that they will take damage for that turn. The Open Palm Monk's tripping attack takes Ki to attempt and still provides an enemy with a Save first. And this list is ONLY for levels 1-4. It grows massively once you start getting higher levels characters.

Each and every one of these has the same benefit as Adv/Dis from Height and Backstab, which means each of those class features and spells are essentially pointless given how much easier it is to just get higher or to walk around a target. This is terrible, TERRIBLE for balance. You are throwing out 6+ years of playtesting the rules and balance of 5e.

Finally, Rogue's deserve special mention due to how their primary class feature (Sneak Attack) interacts with Advantage/Disadvantage. Ignoring the cheapening of the Rogue in general due to every class now having Stealth as a bonus action, Rogue's are not able to use Sneak Attack if they have Disadvantage on a roll. This makes it extremely difficult for a Rogue to use their primary function against any target that is above them. This is terrible for the balance of the class.

Not in the Rules: The rules of 5e to not provide Advantage/Disadvantage due to different in Height. There is an *optional* rule in the DM regarding Facing and what might be called 'backstab' but 90% of the rule (including the ability to use your Reaction to face the target and deny them Backstab) is not implemented, meaning that rule was not used. Anecdotally, I have never, ever heard of any DM using the Facing system because of how much it throws a wrench in the gameplay process.

Possible Solutions: First, remove granting Advantage/Disadvantage for Height and Backstab. As you can see from the list, there are enough ways to get Adv/Dis, BUT they are all balanced by having a cost/penalty associated with using them.

Next, if Larian still wants to incorporate having a benefit for having higher elevation than a target and/or maneuvering near a target, please incorporate the Cover and Flanking mechanics as described in the first linked Primary Topics Link. Cover provides a potential benefit for being higher than your target because your target will not have Cover from your attack. Additionally, you will possibly have some Cover from attacks from below due to the surface providing elevation. Flanking also requires at least 2 allies in melee combat, increasing the risk to those characters for the reward of potentially having a greater chance to hit.

Alternatively, if Cover/Flanking is deemed to difficult or impossible due to the limitations of the DoS engine being used, then replace Adv/Dis with a flat +2/-2 bonus (which is the bonus provided from Cover and Flanking respectively). This makes players still want to seek out sources of Adv/Dis due to their higher mathematical benefit, while also not invaliding all of the listed spells, actions, and class features.