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It is very easy to lose the moral highground in the case of radical change, which often results in the illusion of victimization of the oppressor, and thus the oppressed loses sympathy.
What do you mean by "effective"? If the methods lead to the same result, then by definition they're equally effective, and I would choose the faster one.
Radicalism is subjective. How do you want to define "radical change"?
Some fair examples are the first world countries that had more time to develop by temselves their economy and the colonies that could only get to be part of the economy after a revolution.
Throughout history, movements considered radical by the majority have always been necessary to bring about meaningful change. Being quiet rarely does anything, and even if it does, in the meantime, more people are suffering.
That doesn't imply it's better than the gradual changes.