Both height and backstab advantage are overtuned, but I'd say backstab advantage is by far a bigger offender than height advantage.
Absolutely! Been saying this all along, and it's important to bear in mind with all the focus on height advantage as seemingly the only evil or the bigger evil of the two. Height advantage has ONLY the issue of over-incentivization (incentivization of tactical movement is GOOD however), while flanking has MULTIPLE issues:
1. As you point out, height advantage is not guaranteed for multiple reason, where flanking is a certainty.
2. Height advantage is at least balanced in that it's a tactic the enemy very frequently uses, when they seldom if ever flank.
3. The enemy being totally defenseless/unresponsive to a single attacker makes melee combat unimmersive. Turn-based combat (that I like) is exposed as a sham.
4. The act of flanking is guaranteed, and this in itself is a negative. If something is guaranteed, having to perform a small ritual to get it every time becomes boring and cheesy. When boredom/patience is the only practical limitation of something so significant, it's simply BAD game design. Just like weapon dipping is.
I have suggested the following change in the overly simplistic flanking mechanic:
Melee combatants, even when not their turn to act, will always automatically pivot to threat - unless already engaged in melee with another threat, or surprised. Immersion and balance restored to the system.
Backstab can also just be modded out.
That is well and good; people can ignore or mod away much of this, but will always be aware of the fact they're engaging in self-nerfing behaviour, playing at harder difficulty with no additional reward for the added risk.
The larger issue remains though; the repeated use of resources on "Larian cheese", which too often are unimmersive, unbalanced and clunky implementations, means less resources on implementing actual D&D or story content that has none of these issues.