The multiplier on headshot does not seem to be working as intended. This could be a function of one of two things: either the damage is not calculating correctly, or the display for enemy health is off.
When I see a non-boss enemy with 25%-33% health remaining, and I use Headshot, I'm fully expecting them to go down. 75% of the time, though, the damage they take is lackluster, or fails to eliminate the target.
Maybe it's not getting a critical hit as often as the other skills. I have the damage numbers being displayed, and there is an increase, but it just seems to be under-powered. Without a percentage displayed on enemy health, it's difficult to calculate the damage on Headshot.
As it's his execute, it just seems like it's lacking. Perhaps it needs a tweak such as increased base damage, or a guaranteed critical hit, as opposed to the missing health % modifier to damage. I could see an argument against the auto-crit on the old iterations, where skills could be spammed at will (depending on resource availability), but with limits on the number of uses without a reload, it makes more sense for a guaranteed critical hit (with an elevated chance for a brutal hit).