Right, that makes sense when you put the new advantage sources into context. Which I agree with. I think the only argument for height advantage would be simplicity. It fits into the current UI with advantages going against disadvantages. In a future update with a new UI, it'd be nice to see height go from advantage to behaving like cover. You could even scale it from +1 AC and DEX saves to +5 based on the level of height difference between targets. Could possibly throw range extension in the formula as well. Picture attacking someone on the top of the steps versus attacking someone on top of a wall. Then backstabbing should at least require stealth or flanking. Definitely not the current sidestep or Anakin Skywalker hop we have.
In conclusion, taking away the extra advantage sources and increasing hit chance on saving throw spells would balance it right?
I would be so happy with a scaling +1 to +5 height bonus to AC & Dex Saves. That would solve so many problems. I'd prefer the maximum bonus be +2 or +3, but that's nitpicking details instead of structural.
backstabbing: Flanking is better than requiring stealth imo. Running behind someone, entering stealth, then hitting them is silly.
In conclusion: At Least
either/or. Either take away the extra advantage sources or increase hit chances on saving throw spells would greatly help. This would also still allow people to hit more, which is "fun"