Despite the fact they have the same wording (just different percentages), they don’t work the same.
Unclosing Wounds doesn’t trigger on-hit effects, but it DOES trigger Ethereal Revenant (well, most of the time; see the bug I reported the other day).
Unclosing Wounds reads “Current health drained per second: X%”.
Ethereal Revenant reads “When damage leaves you below half health, summon a Revenant. This effect has a 15 second cooldown.”
Thus, every 15 second, as long as you have the Unclosing Wounds node and are at low health (such as from Exsanguinous + Last Steps of the Living), you will summon a Revenant.
However, Mortality’s Grasp reads “359 ward gained when damage leaves you at low health”. Low health is 35%. I have roughly 163 health out of 1919, well under 35%.