The most important aspect of any cosmetic medical treatment, where art and science combine, is the skill of the doctor.
With my experience of teaching, over the years, I am acutely aware of differing levels of skill in this area. Some doctors, particularly if they are not naturally artistic, can struggle to achieve any level of competence, and certainly would never be expected to reach levels of excellence.
The materials used must be safe, obviously. The materials used in my practice, all have long and excellent safety records.
The skill is in producing a natural appearance, which stands up to the closest scrutiny, but still appears completely natural.