I have been reading alot about this lately.
From what I have read and I think, its ok to scruff an adult cat for pilling perhaps or to temporarily immobilize them for a specific purpose (to carry them into their carrier maybe)...
But all I saw on the subject, stated NOT to pick up an adult cat off of the ground.
You can do it while supporting his body weight with your other hand but never just his neck.
Mums do this to kittens who weigh significantly less and they know what they are doing exactly.
Adult cats do not carry Adult cats around in their mouths by the scruff.
You shouldn't even pick up an adult cat without supporting his body and legs underneath his body, let alone pick him up by his scruff..