That is, picking up a cat by the back of its neck?
According to my vet, no. According to my cats, no.
It calms them down. They go limp, making me able to deposit them in their cat carrier.
(Except currently, one of my cats has an open wound on his neck, just below where I would have grabbed him, so when I took him to the vet I had to use another method to get him into the carrier. He did not prefer that method. Nor did I.)
I would support their rear end if you have to lift them at all; kittens being scruffed by their mama are much lighter than an adult cat, and an adult may find it somewhat painful depending on their weight and how they’re scruffed if you don’t support them.
Ferrets have the same “go limp” reaction when scruffed, and the specialty vet office that I take my ferrets to will do that for keeping them still during an exam.
Can dogs be scruffed? I’m thinking no because I’ve never seen it in person or documentary.
If not how do mother dogs move their puppies around?
Obviously I’ve never owned a dog much less one with newborn puppies.
Well, the mother scruffs the puppies. Scruffing an adult dog is first and foremost a question of ‘may’ rather than ‘can’. As far as I know adult dogs do not go limp in such a situation so if he or she doesn’t like it, you might need stitches. I would also imagine that even for a moderately sized dog it would be painful because dogs are much heavier than cats.
You never met my 27-pound cat, who couldn’t be scruffed without the use of a crane.
Kittens yes, cats no in my experience. And that’s with young cats who weren’t overweight. Or perhaps I was just too soft on them, to me they seemed pained when I did it 
They’re not in pain. They are just annoyed that you discovered their one weakness; That they can’t do anything when scruffed. 
Damn! Can you even find the scruff? And if you can, can you even get a grip on it? 
We got this from a TV vet, years and years ago.
Get some clothes pegs - the springy kind. Older ones are best as they tend to be a bit worn out and thus looser. Grab your cat’s scruff as if you were scruffing them, clip on a couple of pegs. Ideally your cat will just crouch and not move, as if you were really scruffing them.
It worked for one cat. He crouched, then keeled over sideways until mum took the pegs off. The other cat shot off around the house at a million miles an hour.
I will still scruff my cat if I need to get her somewhere fast, and can’t be bothered battling the claws (if she’s escaped out the front door, for example). But I always support her rear end when I do it, because even though she’s not a particularly huge cat, she’s still heavy enough that she’d hurt herself if I didn’t.
Experiments with cats, great stuff. I once had a cat contented enough from being stroked that she lay on her back on a bean bag for a few minutes looking to all as if she was hypnotised.