What if someone has very low stress levels, what's the worst that can happen?

I don't really think there's such a thing as a stress level that's "too low". There's just being stressed, and then if you're lucky enough, no stress. It should mean your body is producing more stable and healthy amounts of dopamine, oxytocin, serotonin, whatever, and your blood pressure is more stable.

I guess there's such a thing as being so indifferent/unaffected by events to the point of disconnection, demotivation or lack of willpower to do anything. Feelings associated with depression. I don't think that would directly affect your heart or blood pressure, but it may affect your desire to take care of yourself, which can affect your health in many ways in the long run. It may affect your social life as well.

Likewise, there's also such a thing as being TOO excitable and or "too happy". There's plenty of documented cases of people who laughed themselves into a heart attack, or were so shocked by a touching event that their blood pressure skyrocketed and they had to take a seat. Straining your body with too much of any kind of emotion can cause problems, really.