It hasn't been done before that we know of, and first time procedures are generally thought to be risky.
The spinal cord would have to be cut in order to transplant a head; when spinal cords are severed, it causes paralysis, and thus far medical knowledge hasn't figured out how to repair a severed spinal cord, so doing it on purpose would definitely cause paralysis and most likely be a death sentence.
It is the brain which controls absolutely everything that happens in a human body. Neurons are microscopic in size, and attempting to re-attach them correctly would be orders of magnitude more difficult than any reattachment process we have tried previously. What means of attaching ends of severed neurons could there possibly be? Any suture material would have a larger diameter than the neuron, so how's that going to work? Superglue wouldn't work because it would be a barrier to the transmission of the electrical/chemical signal from neuron to neuron. Also, since it is the brain which governs the operation of all bodily functions, and the brain needs a constant supply of oxygen or the brain cells die, then how can blood be supplied to the brain that is being transplanted? It is one thing to put a heart on a heart/lung machine during an operation, but no technology exists to supply a brain with blood if it is not attached to a body that is pumping blood to it. Also, for operations requiring use of a heart/lung machine the duration a patient is on the machine even for cardiac bypass surgery, is in the vicinity of an hour. The time it would take to reattach microscopic neuron endings would is incalculable. The average human brain with spinal cord has about 100 billion neurons. It just seems too daunting to mess with that.
Also, the spinal column is encased so that there is spinal fluid completely surrounding the nerve fibers and acting like a cushion that protects the spinal column from injury. If you are transplanting a head, that opens the spinal cord cavity up, risking infection and injury to the spinal column.