And assuming a locking nut or two is involved, why wouldn't it be OK?
Using an existing screw saves a few cents on drilling a hole and installing a new one. As long as there's good electrical contact to the chassis, there's nothing wrong with it that has anything to do with engineering or safety.
Because the physical stresses on the chassis caused by weight and vibration of the transformer can cause the nut to come loose or otherwise compromise the integrity of the electrical connection.
I have seen vintage Fender amps (steel chassis mind you) where they used a PT bolt for power amp grounding point -- the blob of solder cracked and the keps nut loose.
Not worth saving a few cents IMO.