Are there cases where Reiki can be harmful?

The person who gives Reiki energy is not actually the one who does the treatment, they are just a channel for the Reiki energy to flow through them.

They are not the one who heals, it is rather the Reiki energy that does the healing. The Reiki therapist leaves their personality, their ego outside the room. The reiki energy has got its own intelligence, it knows where to flow; it follows the balance of the nature.

The reiki therapist leaves their mind outside and offers their body as a vessel for the use of the energy. During a Reiki session, one never receives too much or too little reiki energy.