here are my two cents on the matter:

advantage for high ground AND disadvantage for low ground are just too much of a disparity (basically +/-10 between the dice rolls of two combatants on differing height levels).
i could live with adv for (a reasonable i.e. >3m or 10ft) high ground WITHOUT also giving disadvantage when shooting or casting from low ground.
or give a flat +2 bonus and -2 malus for high and low ground respectively.
maybe this would be a good compromise, engaging with the verticality of the world is still encouraged and helpful but not as blatantly "THE thing to do".
best case scenario would probably be a "custom game rules/difficulty settings" menu, so everybody could adjust the game to his/hers/their liking, the hurdle here being the AI, which would need to be aware of the rule settings and adapt to them.