After some experimental testing, once the height reaches a certain threshold, fall damage will inflict enough damage to kill you regardless of how much HP your units have. I tested this with three different characters with different classes all in the same location and height.
- Taion - At lv79, he had 2973 HP. Fall damage dealt 4546HP
- Noah - At lv79, he had 8001HP. Fall damage dealt 12194HP
- Mio - At lv79, she had 3320HP. Fall damage dealt 5004HP
Calculating out the character's max HP and the fall damage that was dealt, all three characters took ~165% damage. Based that information, it appears that fall damage deals a percentage of the unit's max HP, which increases in relation to the height. Based on that, once the height reaches a threshold to deal 100% HP, your unit will not survive no matter how much HP they have (and as you can see in the video, the fall was not all that high comparatively speaking to the rest of the world.)