Like any group of humans, there are power structures and edge cases that can lead to horrific outcomes. Giving the person that posted the warning the benefit of the doubt, I think what they are saying is that "Rationalist does not necessarily mean positive for humanity, nor even no harm for humanity". This holds for all religions and religion-like movements, of which Rationalism, in this sense, is one.
Like any group of humans, there are power structures and edge cases that can lead to horrific outcomes. Giving the person that posted the warning the benefit of the doubt, I think what they are saying is that "Rationalist does not necessarily mean positive for humanity, nor even no harm for humanity". This holds for all religions and religion-like movements, of which Rationalism, in this sense, is one.