Wood mulch is often used as a ground cover, but it doesn't hold up well on slopes, or when exposed to high winds, heavy rain or run-off. Due to it being so light, it blows out with wind all over the place. Everywhere but where you want it to be. Likewise with rain or run-off, floating down into the culverts or street drains. It can be fairly normal to have to keep replacing it as frequently as every 6-8 weeks, which adds up in labor costs, even if the wood mulch itself is free or created from tree trimmings in a wood chipper.