I don't think the Harlequin's kiss is all as powerful as it sounds to be honest. 100m of this wire in a device the size of someone's wrist means it will be a very very thin wire, razor edged or not. I can understand such a weapon being lethal to humans, eldar, tau, and probably orks.
But astartes physiology is a step above all these. Remember, the device works optimally when it is actually punctured through the victim's skin so that the wire unfurls inside and hits the internal organs and arteries directly. But even then, we're talking varying degrees of density here, organs, fluids and bone will all offer different levels of resistance no matter how sharp something is, especially when that something is so small. A non puncturing hit probably only results in superficial external cuts.
Tyranids are on a different level altogether, most of them have endo AND exoskeletons. I believe the above mentioned Hive Tyrant would easily survive.
For that matter, I believe the Primarch will also be fine. Wounded probably, but not fatally. Unless you're gonna hit him in the eye socket with that thing. :grin: