So I've been looking up what the side effects of fast regeneration would be, it's a bit interesting.
Given just how fast the Immortals can heal, if someone was able to heal in real life as fast as some of them appear to in the movies and TV shows, it would use up calories and generate heat at the wound site.
However, where it really gets interesting with the speed of the healing, is just how that increases the calories used and the result of the heat generated. Something like a papercut on a regular mortal takes a few days to go away, an Immortal might have that vanish in seconds, but a deeper cut can take a few weeks to heal. How long does a deep cut take an Immortal to heal? Maybe a minute or two?
The amount of heat generated in healing a deep cut that fast, apparently, would cause the wound area to catch fire and maybe explode! Even if it took ten minutes to heal, the heat energy generated would end up leaving the area cooked.
But the same would be true of a papercut healing in 1 second too.
Now I doubt the writers had any idea of this, and given this is Google's AI giving me these details without any citations to double-check, I do have my doubts, but if it is true that this would happen, then clearly the Immortals would have to have some other biological or magical or supernatural element in play to counter it.
It certainly puts a different look on Dr Who's regenerations.