Height does bring advantages, mainly increased range and it is often far more easy to get behind cover. If the height difference is very pronounced, then I just need to duck a little bit, and you won't be able to see me anymore.
So the Advantage/Disadvantage system in BG3 currently is just way over the top. But a lot of people have already pointed that out.
My personal preference would be increased range with bows and crossbows, not with spells, and that's it.
+1 (If I'm looking for an ideal situation for the game)
I'd like to supplement this with some more minutiae:
Starting with (A) Height wouldn't provide AdvantageFOR/DisadvantageAgainst
(B) going beyond normal range is already in there, add in height extending the "normal range" for ranged weapons.
having a proper climbing speed (Ladders, etc.)
rough terrain being applied to ascending steep angles
Ranged attacks no longer being at disadvantage because an enemy is ~3-4 meters away (5 feet is fine for disadvantage)
Add in +2 to hit
^ This would be a healthy baseline for high ground benefits.
Higher ground would give a better vantage point in combat and enemies would have to expend additional resources to ascend. Also agreeing with applying a simple +2 to hit.
(Side note) Personally I'd rather we had proper Dodge or Prone actions than getting a bonus to AC for high ground. It would give the player more risk/reward options.
The core intent of 5e is that bonuses don't stack. The player takes the greater value that they've received. You see this throughout 5e. Examples: Temporary Hit Points from multiple sources, and Tortle's Natural Armor with Barbarian's Unarmored Defense.
Applying 5e's logic: if a player gains Advantage, Advantage would override a +2 to hit and not stack.
