If a theory purports to explain everything, then it is likely not explaining much at all.

The Stoics thought that a fundamental insight into human life is that some things are up to us and others are not, the famous dichotomy of control. Up to us are our judgments, decisions, and actions; everything else is not up to us, because it is influenced by external factors.

Often we can change things, and a realistic attitude - including envisioning worse case scenarios - actually helps to accomplish that change. But if you truly cannot do anything about something, then why on earth would you want to make things even worse for you by falling into despair? It seems like adding a self-inflicting injury to the already existing one.

Death itself is a natural occurrence, it is unavoidable, and the Stoics thought that part of philosophical practice is to get comfortable with the unavoidable, learning to face it with courage.

... it is because one can build a compelling set of arguments - informed by science and thoroughly compatible with it - that to believe in anything despite the complete lack of evidence is, in fact, irrational.

I believe Stoicism can help anyone who takes it seriously and begins to practice it. It will help them flourish because it will provide them with a compass for navigating life, a general, flexible, framework to set priorities, and a set of techniques to achieve serenity and equanimity.

I don't see any Stoic practice as problematic or risky, but I would advise to engage in extreme versions of the negative visualization exercise only if you are an advanced practitioner. The negative visualization is a meditation during which you visualize, slowly and deliberately, something bad or discomforting happening to you.