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.
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?