Mantras bear a relation with respect to the person who is chanting them. Accordingly they are classified into
The fourth category called Ari mantras are enemies of the person who is chanting them. Which mantra could be an ari mantra for the individual depends on the syllables of the mantra and astrological particulars of the individual. This can be identified only by someone who is adept in mantra sastra.
Chanting an enemy mantra can lead to destruction .
This is highly individual specific. Public deeksha ceremonies do not take this into account. People who go around advising " Do one mala japa of this mantra or that mantra" do not know this.
Moreover, there tatvas, gunas, gender etc. associated with mantras. Careful consideration of all these is required before taking up mantra chanting.
Better to stay away if you are not under able guidance.
Sincere prayers and simple stotras do no harm.