Of course some variations (cobra, and various others.) but they all have a common trait. 15 hit points, so they are quite fragile. However, they have one deadly trait. When they bite a villager they unleash a deadly venom. Even if a villager has loom they will die from the poison effect if he does not go into a town center or gets a monk to heal them.
This could also damage rushes because you could lose a unit to a random snake biting one of your units. The snake unit is small and hard to see, and archers don’t automatically target them.