It is often claimed that certain actions - usually in the field of medical and biological science - amount to 'playing God' insofar as the foundations of life are manipulated and synthesized artifically. However, isn't this merely a rhetorical claim given the impossibility of humans acting as gods? Also, what strength does such the 'playing God' claim have against the irrefutable claim that all knowledge demands risk, and an initial ignorance to provide the impetus for the research?
You are right that saying that someone is 'playing God' is merely a rhetorical claim made by those who oppose the kind of actions that the person is doing are contemplating doing. Since, "given the impossibility of humans acting as gods," as you say, it is a way of saying that you should not do the action because it is inappropriate for a mere human to act in a way that is appropriate only for a god. Insofar as the claim has any force, and it does not have much, it is a warning about making fundamental changes without very carefully considering all of the consequences, long term as well as short term, of making these changes. Many actions have massive unintended consequences, especially those that involve changing how we deal with life and death matters.