Mathematically, advantage and disadvantage are most impactful for middling DCs. Averaged from 1-20 results it only comes to a 5% difference.
It's easy to negate because sources don't stack. You don't get double advantage. You just have advantage. All it takes to negate is a single source of disadvantage.
Now, think of mid to high level monsters and you'll see it really won't mean much at all over the course of the game as a whole. It feels OP only because of the combination of low level and easy enemies. That's it.
Not sure where you learn maths or probabylity or if you really know what advantages and disadvantages mean but you're totally wrong. The average advantage on a D20 roll grants +25% to hit while the average disavantages decrease your %to hit by 25%.
That's exactly how it work if your character is level 1 (proficiency +2), if your build is "correct" (modifier +3) and if you fight against ennemies that have an AC of 15 (D20 roll >=10). You can check in the game.
My knowledge of mathematics is probably not much greater than yours, but it seems to me that to reach the 5% that comes out of your hat, a common lvl 1-3 character should fight against ~AC 23 ennemies. There it wouldn't be very much impactfull... but the entire game would suck because you'll miss except if your D20 roll is >= 18
Now about "negate" advantage and disadvantage, it looks you'll stay very vague.
Just one exemple... What can your ennemies do to avoid your backstab when you can cheesy jump and attack once (or more if you're a fighter lvl 5) each turn from it's back ? Is he going to cheesy disengage and always try to reach your back to keep the balance ? Or is there any other consequences of your cheesy jump and cheesy backstab(s) to keep that balance ?
I'd really like to have your real exemples of how "balanced" advantages/disadvantages is and how "easy" it is to negate. I guess easy means we have many many possibilities. If not... maybe "determinism" was finally the right word.
