well metal is stronger than food, most metal anyways. technically when you put an aquarium on a wooden stand, the bottom of the tank doesnt get any support, but the built in brace, so i dont see why metal would be worse, other than if it cracked, it would be on your floor imeadiatley instead of a few hours with a solid wood stand.
(btw when i say wood stand, i mean stand, not a frame)
others might have other reasons im unaware of...
"Atrocities are not less atrocities when they occur in laboratories and are called medical research."
George Bernard Shaw