I think, canonically, the reductor curse only works on inanimate objects -- at least, that's all it's ever used on in canon, and you'd think it would see some use in combat if it was at all effective for that. Likewise Diffindo
: if it cut living flesh, Snape wouldn't have needed to invent Sectumsempra
to achieve the same effect.
One of the laws of magic in the Potterverse appears to be Vitalism
; certainly the concepts of Avada Kedavra
(a curse that kills without causing any detectable injury) and Horcruxes are consistent with that belief, and the use of different spells to achieve the same effect on living vs. non-living things (not just the afformentioned blasting and cutting charms either -- healing magic doesn't seem to use the same spells as are used to repair broken inanimate objects).
All available evidence supports the conclusion that vitalism is not
valid in the real world, but the great majority of people remain vitalists, either because they are unaware of the evidence supporting materialism, or because they reject it for emotional reasons. J. K. Rowling, as an Anglican Christian, is presumably a vitalist herself.