I agree it's massively over tuned. Here are other considerations:
Some players are really attached to high ground Advantage (not even realizing that low ground disadvantage is a thing). So I'm still proposing using "Combat Advantage" from 4e, where high ground would give a +2 to hit.
Low ground Disadvantage somewhat acts as cover, but still feels silly since most trained riflemen/archers can hit a target on higher elevation. I'm somewhat mixed on removing it entirely (after discussions with other forum members), but it's either players don't notice it exists, they dislike it, or they think it should be a -2.
Lastly, high ground Advantage & low ground Disadvantage make no sense for spells.
Remember, the "-X" I say here is actually a "+X" for the target's AC.
We agree that giving advantage/disadvantage is strong. But I don't see why arguing about +2, although I prefer that to an advantage.
The reasons I gave are enough for high ground to have its advantage, but think about it, a +2 doesn't make sense, but a -2 or even a -5 makes sense, I'll explain.
A -2/-5 would make sense, as it's kind of a cover, like in the D&D rules. Why would someone shoot from high ground and not use their speed to get out of the enemy's line of sight and go into full cover? means he wants to watch what happens in low ground, so he's at an angle where only his torso or head appears, to watch. That would be a half cover or three-quarters cover. This rule could be measured by the distance the shooter is from the edge, or else by the height of the shooter in relation to the opponent. I prefer it to be the distance from the edge, because that wastes speed and makes more sense, it's more realistic, but in that case, a coverage indicator would be needed, which makes it a little more complicated.
Here I'll leave the D&D cover rules for you to see:
Cover Walls, trees, creatures, and other obstacles can provide cover during combat, making a target more difficult to harm. A target can benefit from cover only when an attack or other effect originates on the opposite side of the cover.
There are three degrees of cover. If a target is behind multiple sources of cover, only the most protective degree of cover applies; the degrees aren't added together. For example, if a target is behind a creature that gives half cover and a tree trunk that gives three-quarters cover, the target has three-quarters cover.
A target with half cover has a +2 bonus to AC and Dexterity saving throws. A target has half cover if an obstacle blocks at least half of its body. The obstacle might be a low wall, a large piece of furniture, a narrow tree trunk, or a creature, whether that creature is an enemy or a friend.
A target with three-quarters cover has a +5 bonus to AC and Dexterity saving throws. A target has three-quarters cover if about three-quarters of it is covered by an obstacle. The mighty obstacle be a portcullis, an arrow slit, or a thick tree trunk.
A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such as a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.