The strong version of the adversity hypothesis might be true, but only if we add caveats: For adversity to be maximally beneficial, it should happen at the right time (young adulthood), to the right people (those with the social and psychological resources to rise to challenges and find benefits), and to the right degree (not so severe as to cause PTSD).
Jonathan Haidt, The Happiness Hypothesis: Finding Modern Truth in Ancient Wisdom
adversity, strength