Many interventions from different theories are fine to learn superficially, but ones that have potential for intense emotional reactions/expression aren't appropriate for superficial use and are on the border of being outside the bounds of therapist competence without proper training.
"If you plan to practice Gestalt therapy (i.e. Empty Chair/2-Chair), we strongly advise that you pursue advanced training....Getting caught up in the excitement of pushing clients toward change and emotional breakthroughs commonly associated with Gestalt therapy (as illustrated via many of Fritz Perls' stage demonstrations) can result in unnecessary negative outcomes." ---Sommers Flanagan & Sommers Flanagan, Counseling and Psychotherapy Theories in Context & Practice
I agree that clients shouldn't be harmed in the process, which is why it should be used with caution and proper training. Trauma, suicidality/severe depression, self-harm, and psychosis are just some issues in which this type of intervention would be dangerous and harmful to use with, but proper training would provide the right level of judgment as to when to use it/not use it and safe guidance through the process for appropriate clients. I've heard of an individual experiencing full on panic attacks/flashbacks after this intervention with an improperly-trained therapist who then had no idea what to do to calm the client down. While this is an extreme example, it serves as a cautionary tale not to practice outside of the bounds of competence.