I wouldn't say Larian's system removes the value of the X amount of spells, but means for them to be used differently.
Using those spells is definitely a sub-optimal strategy. You can use them of course but there's always something better.
The problem with backstab is the same but the other issue you're talking about comes from disengage, not jumping. Adding a dexterity check when you disengage mean you can't ever just disengage.
I fought
the myconid colony, and the hob goblin
jumped onto a mushroom and used blur. The only way I could fully break disadvantage in that specific comp/situation was to use truestrike and move to equal ground. THAT is a king of the hill ending, but if I had any other abilities to provide advantage, I could very easily have attacked from beneath him. I can't think of many fights where I could just walk right in and destroy from high ground, either. I would often be uprooted or threatened for even trying. Every fight where high ground won me the fight was after I pre-planned and set up for an encounter, which would require scouting/prior knowledge. I would SPECIFICALLY move to locations that made enemies walk the longest way around. Considering the amount of people complaining about hit chance and fights being too difficult, I think it was the smartest move to lessen the learning curve and appeal to a broad audience.
I'm 88% sure there is no stacking advantage. The rolls in the combat log detail only 1 (one) advantaged attack roll, and it details everything else pretty well, so no real reason to believe otherwise. It's more of a, "fight for one advantage" deal in this game which includes but is not limited to king of the hill style gameplay.
I personally am also a fan of the one button jump/disengage BA
