Fines create dust and tend to pack which inhibits drainage. Sand can also contain small gravel or stones. Unlike clay, sand has more strength when it is wet, in fact, it often becomes too hard. When it is too dry, it loses its binding forces and collapses. However, as the name implies, this introduces a dust problem.
Wood products add organic materials to the footing which keeps footing alive, springy.
Wood products tend to break down quickly when they are dry as they shatter and easily become wood dust. However wood footing that is constantly wet might decompose via rotting. Wood footings made of smaller particles tend to break down much more quickly than those of larger particles. If kept regularly damp, wood footings last longer. However, wood footings can become slick if they are more than damp especially when a large constituent of the wood footing has been pulverized to wood dust.