Why do some Christians say it wrong to "play God" in medicine?

God is all good. He came to earth as Jesus Christ to die for our sins and give us an example of a perfect life to attempt to follow. Medical technologies take many forms. Some, for example antibiotics, are considered unambiguously to be a moral good. Others, for example gene therapy, are considered questionable because of their nature, with some claiming them to be "Playing God" and therefore sinful. I interpret this to mean that they are bad because they are more like the sort of thing God does. What is the logic here? It seems to me that anything that is like what God does must be inherently good, because that is what God is.