Grey Nymph can't compete with Bone Dragon in Monodeath rush, nor can Death Nymph compete with Ivory Dragon. Death Nymph can compete with Bone Dragon since they have the same attack:cost ratio, but rushes still tend to pack Dragons for the higher total offense. The Nymphs are used in rushes if you want CC resistance since it's the only Death creature with good HP, but the more you pack the more to go from rush to domin. Additionally, Monodeath rush almost never worries about keeping its creatures alive since it almost always packs Poisons+Arsenics.
If your strategy relies on Afla, as with all Nymphs it's only 100% reliable to pack the Alchemy card instead. The Nymph does, however, fit perfectly into a Light/Death domin/stall that doesn't have to Afla to win.