Yes as I said on the blog I wouldn't use this technique for all my WM armies and not even for all the units in this army.
The Spirit Hosts are white undercoat and Aethermatic Blue for instance

I did the Zombies in a similar way to the bats though, just started with a dark brown undercoat

No-one notices the unpainted/in shade bits at gaming distance - the only question is if you as the painter can live with the knowledge you haven't done it "properly"
Yes in reality the Nuln oil helped - the photos are fairly crude badly lit phone pictures, the Nuln toned down the sharpness of some of the drybrushing and also tied it all together.
But then like all these things it's subjective, add a wash, don't add a wash, paint the ears, don't paint the ears - it's all about what *you* are happy with.
I've got some BoFA Dwarfs based and undercoated black - metal drybrush for them is definitely the way I'm going with those!