I realise that trying to apply real-world physics to 40k is an exercise in futility, but this is more of a matter of common sense (at least I think it is)
Eldar make a lot of use of monofilament weaponry (enough to get a section in the fluff armoury of the 4th ed Codex) and the reported effects seem to vary widely.
The weapon mechanism is generally based around creating a web of very thin, high-tensile "wire" and draping it over the target.
In the 3rd Ed codex, this caused damage through the target struggling against the "web", causing the wire to bite into their flesh - because the wire was both thin and strong, it could cut deep into flesh with little effort.
This seems pretty reasonable - anyone who has walked into a spiderweb will know that it can "cut" into your skin a surprising amount (although it isn't strong enough to break the skin)
With the advent of the WD nightspinner rules, the fluff mechanism was changed to "web of wire is launched high into the air and falls to the earth - it is so sharp that it cuts clean through anything in the way"
The problem I have with the "new" (nightspinner) fluff is two-fold. Firstly, the monofilament is both extremely light and would have a very large surface-area-to-volume ratio, meaning it would fall extremely slowly, and with very little force behind it. If it encountered an obstacle, it would simply drape over it. Secondly, the most likely cutting mechanism for monofilament would be shear, which requires lateral movement - to give an example, it is possible to climb a ladder made of swords with bare feet, provided that care is taken to lower the foot directly onto the cutting edge; at the same time if you drag a feather along the edge it will cut in two under its own weight.
A similar problem applies to the dreaded "monomolecular whip" of other science fiction settings - the whip would be so light that it must have almost no kinetic energy; it would be like trying to use a spider's web as a whip.
Or have I made a massive error somewhere in my thinking?