It's not entirely anecdotal. There's research that shows meditation can hasten the progression of mental illnesses like suicidal depression and schizophrenia. For example, suicide rates spike among neophyte practitioners of meditation, and those with latent schizophrenia are more likely (i.e. earlier than others) to experience their first psychotic break when beginning meditation. AFAIU, Buddhist teachers, pre-modern and modern, were and are taught to pay attention to warning signs of people experiencing pathological mental issues brought on by meditation. And while I haven't read of it specifically, I would think the phenomenon would also be found in, e.g., contemplative Christian communities (e.g. monastic communities or contemplative retreats). None of this implies meditation is bad, just that it's important that people learn meditation techniques in a supportive, non-isolating environment (i.e. with a well-trained teacher or experienced community) so that the small fraction of those who might respond negatively can be helped. This type of environment would indeed tend to be found within traditional religious institutions and communities, as opposed to following a YouTube video or website howto at home.
It is sort of known that meditation can be a double edged sword, it is a tool and like any tool it can be used badly which is why traditionally you would have a teacher.