Strongly Agree with OP.
Advantages for high ground are too drastic. I am unsure of the formulae that Larian uses to determine the hit% in these situations, but it is clearly out of line with 5E rules. Case in point:
Wyll and Gale have identical ACs (14) and spell attack modifiers (+6) Will is placed on high ground and targets Gale with Eldritch Blast and has a calculated 84% chance to hit, with no other modifiers.
Conversely, Gale has a 42% chance to hit Wyll shooting from low ground with Witch Bolt. Again, no other modifiers. See SS's below
https://drive.google.com/file/d/1vEw11hRiLWmVeViQNtdTgKm9nUmP-k4u/view?usp=sharinghttps://drive.google.com/file/d/1Up_DZYeQubnQFcD4vOwsRen_J6kC-V--/view?usp=sharingProposed solution: Remove negative/disadvantage modifiers from low ground. Continue to allow high ground to be positive/advantage. Or, at the vary least, make both gameplay options that can be enabled/disabled at user discretion.