@Zven93 there's also absolutely nothing stopping the enemy general retreating 1 mile, then sending the army back while bandaging up his hand, where he CAN'T be sneak attacked and is out of danger.
Harrying the enemy is a task that's vital. And an army without a headquarters will find it very hard to re-establish lines of command, especially when they're still being attacked.
You are correct, it's a gamble - but so is letting them go unopposed. Destroying the enemy's command structure while they're in full retreat is almost certainly going to solidify the flight of the enemy.
Still, we'll see how this plays out. It's a story, anyway.