They bounce off the surface without grinding it as sand does. And they are biodegradable. Of course, you do have to wear a helmet and face mask while running the thing. These sculptures have a hard life. Their linseed-oil coating deteriorates, turns brittle and flakes off in the sun.
Cables snap in the wind.
Thousands of touches can cause bronze to lose its patina, because the oils in people's skin are acidic, and they can actually etch into the metal surface. Kids start to climb on the pieces, or not knowing any better, parents put them up there to take a picture. They think the garden is a playground. But even tennis shoes have pebbles in the soles, and they scrape on everything. When those break in high winds, the whole sculpture has to be laid on its side for repairs.