It's really not hard at all to die once you're knocked unconscious.
"If you take any damage while you have 0 hit points, you suffer a death saving throw failure. If the damage is from a critical hit, you suffer two failures instead."
"Unconscious Condition: Any attack that hits the creature is a critical hit if the attacker is within 5 feet of the creature."
In other words, if a dragon has three attacks or more (like an adult dragon), and it knocks you out with the first and then closes with your unconscious body to complete the remaining attacks, the other two attacks will kill you. No save, and their attacks are all at advantage once you're unconscious as well.