According to a review of over 40 years of research that was published in 2020, the most common adverse effects are anxiety and depression. These are followed by psychotic or delusional symptoms, dissociation or depersonalisation, and fear or terror.
There is a very specific term to describe a very specific dark phase one supposedly goes through when meditating too intensively for too many years. Like brain-altering dark. Can't remember the name so feel free to fill me in here...