Some people who are brain-dead are really not dead, but does this mean there is no such thing as brain-death, or just that some people are wrongly diagnosed as brain-dead? Some people before who were announced dead ended up being buried alive.
The Church, from what I understand, accepts the expertise of doctors, and many doctors do believe brain-death is death. Others may disagree; but no one can say there is any clear consensus opinion. Therefore, I believe there is freedom in these matters, for now, and if a doctor says someone is brain-dead, they are dead.