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.
The bonus on an advantage is substantial. It's +25% on a 50% chance roll. And so it should be acquired by using resources instead of just placing your character behind a target or literally moving one step up on a stair.
It they are so scared players would be disappointed to miss too often, then they should give everyone advantage by default and use +/- 5%, +/-10% bonuses to adjust in case of backstab or elevation.