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.