It is important to note that while the race is purported to be a recreation of the famous "serum run" delivering vaccinations to Nome, that run -- a mission of mercy for people -- was also far kinder to the dogs. That race was run in relays by different teams of dogs. As a human and dog endurance test, the Iditarod bears no resemblance to the original run.
Forcing dogs to run the entire distance is inherently dangerous and cruel. Dogs suffer. Since the race began decades ago, more than 140 dogs have died during the event -- from heart attacks, pneumonia, muscle deterioration, dehydration, diarrhea, and spine injuries. They are impaled on sleds, drowned, or accidentally strangled.
While the industry likes to say that sled dogs love to run and pull -- which is true -- this is primarily a function of living most of their non-pulling life crowded into small kennels with no state management or oversight. They might be tethered on short chains at all times, unable to play, forced to sit, stand and lie in the same small area in which they eat and defecate. Commonly, dogs who can longer pull competitively are dumped are killed.