Today morning I sent similar feedback via Larian's formular, with special mention that high ground rule have same power level as 9th level spell Foresight: You touch a willing creature and bestow a limited ability to see into the immediate future. For the duration, the target can't be surprised and has advantage on attack rolls, ability checks, and saving throws. Additionally, other creatures have disadvantage on attack rolls against the target for the duration.
From all what I've seen in my 100 hours in-game the backstab and high-ground rule is the worst modification which pretty much invalidates half of D&D rules.