Slingers have an attack bonus of 2 against archers and also have 2 armor when dealing with enemy archers and siege artillery. They also have an increased attack against walls and towers. Ships, infantry (assuming the slingers don't run for it) and cavalry can all take them out very easily. They are easily overpowered by Bronze Age units and are often obsolete by that time. The Hittites and Assyrians don't have Slingers.