I can see this, definitely. In PnP, how affective your spells are are based on you primary stat used for that spell. (Int for wizards, Wis for clerics/druids, Cha for warlocks, etc) This is taking into consideration who well your wizard is able to memorize and recall the spell, how close your cleric is to their god. Things like that. This is why high level practitioners, who have invested time into learning their craft, are immensely powerful. Now, when you start taking into account height advantage into the mix, you might as well throw all that skill out the window. When even a 18 lvl wizard will fail to hit a creature, simply because they are on a ledge above them is, to me, absurd. Now, cover, yes. A creature can hide behind a brick wall to avoid a spell (assuming the spell is not an area of affect), and that is reasonable, but those mechanics are also in PnP.
I hadn't even thought of this at all but now you mention it, I'm actually quite shocked at the possibility of a high level caster failing to hit enemies above them. I truly hope Larian have considered the implications of their insistence on high ground combat mechanics.