Dr. Canavero claims to have performed the operation successfully on a monkey and a dog, which are not humans, but it's better than nothing. What you can't prepare for on monkeys and dogs, though, is the human experience of living perched atop a body not your own. Spiridonov has no memory of walking, so he'll have enormous physical and psychological hurdles to overcome should the procedure prove even moderately successful.
You don't need to be a doctor to understand that this is an extremely risky procedure. Even if they reattach the spinal cord, there's a risk of rejection, brain damage, and myriad other pitfalls that a team of 80 surgeons are trying to anticipate and avoid.
Some outside scientists have criticized Drs. Canavero and Ren for promoting bad science, but if you were Valery Spiridonov, and knew your condition would kill you, the minute chance of successful, groundbreaking surgery might be worth the risk.