On the advice of Stabbey, I'm reposting something I wrote earlier as well as a link that Stabbey provided:

Originally Posted by Isaac Springsong
For a sense of how game breaking that (Adv/Dis from Height and Backstab) is, and how much it throws the rest of the game out of balance, here is a great example:

The most powerful resource in all of 5e, and generally in all of D&D across 40+ years of balancing and development, is a *9th Level Spell*. That is the highest magic a player can cast, reserved for the strongest spells. You can literally wipe out an entire city with a single spell. You can alter the fabric of reality. You can *stop time* with it. It is the pinnacle of player power. What is one of the most popular uses of a 9th Level Spell?

Foresight. A spell that does nothing more than give you Advantage on everything for a day, and Enemies Disadvantage against you. That's it. That's all it does. That is how strong Advantage and Disadvantage are in 5e with its Bounding Accuracy system.

Here is a link to statistical modelling of Advantage/Disadvantage.

As you read this, please ask yourself....which option will require phenomenally less work and time: to rebalance 56+ spells and class abilities, or changing literally two mechanics to instead be static modifiers?