I think extremism is obviously very clearly unhealthy by definition.
I have yet to encounter a single scenario in my entire existence wherein the fully-to-one-side-of-any-spectrum-you-might-imagine "extreme" point of view on how to solve any particular problem has been the best, most useful one (to my problem-solving brain, at least).
We humans tend to do this thing where, when we're presented with evidence that a previously-held position might be wrong or harmful, our opinions massively pendulum-swing the other way and we have a hard time finding clear, nuanced middle ground.
"Turns out sunlight gives you cancer, so stay indoors all the time! Avoid all sun exposure at all costs!," we say for a time. But no, it turns out that even though it is technically always doing harm to your DNA, sunlight is inescapably necessary for life and healthy immunological function.
Life is complicated, and extremes are simply shit for problem-solving.
Are you familiar with the Overton window? According to it, you will never see successful extremes because what is considered extreme changes before the world does. I think you can find plenty of examples of historical extremism that is now normal (and "healthy").
My use of the word "extremism" may have been slightly misleading; by "extremes" above, I don't mean "ideas or movements popularly considered extreme", but rather literal extremes in the space of possible solutions to a given problem.
