Eleonore Stump says suffering is chemotherapy for spiritual cancer. But a basic objection to the soul-building theodicy is how it can backfire inasmuch as tragedy makes some people worse rather than better. Makes them bitter, cynical, morally hardened. It can be counterproductive, by driving some people away from God.
However, that would only be a defective strategy if God intends all people to benefit from suffering. But if God's aim is not to nurture soul-building virtues in everyone, then it's not a failure if that doesn't transpire in every case.