Like with many things, it’s often over exaggerated and with little evidence to back it up.
There's enough that canine veterinary manuals provide a chocolate toxicity calculator, to ascertain whether treatments are necessary and whether it requires emergency intervention.
Around 8g milk chocolate (0.9g of cooking) per kg of bodyweight seems to be the real danger point, but it's also highly dependent upon other factors too. The scary part is how little cooking chocolate is needed before you get the returned value of either 'Death possible' or even 'death probable'.
Chocolate toxicity can also result in seizures, tremors, irregular heart rate, heart attack or internal bleeding, as well as death. Usually these symptoms present soon after a period of hyperactivity, but they may occur out of your sight or while you're not around.
Plus chocolate is often full of other harmful things too, like preservatives, sugar and fat - This last one can cause pancreatitis, which is another life-threatening condition and extremely painful even if it doesn't kill.
Also, repeated exposure carries a cumulative risk of poisoning, as well as other conditions.
You'll always get tales from people who knew dogs that ate XYZ and never died. Doesn't mean all dogs are utterly immune, so why would you want to risk it?
However much chocolate your dog just ate, it's always worth simply phoning the vet for their advice.