A placebo is by definition something that has no effect at all. Any improvement in the state of someone taking a placebo must be attributed instead to something other than the placebo. However, patients will attribute their improvement to the placebo. This is the placebo effect.
No doctor would literally say that a placebo is a "powerful drug", since it is by definition completely inert. However, they may say so in a figurative manner to mean that patients will claim effectiveness of ineffective treatments due to the placebo effect.
Treatments that rely on the placebo effect have two major negative aspects. First they may prevent people from seeking a truly effective means of treatment. Second, they take money for providing a treatment that has no actual value.