If you aren't in the top 1/3 of your medical school class and don't have above average board scores, your chances of matching into derm are super low. Also, having an MD/Ph D helps.
Radiology is slightly less competitive than derm but still if you weren't in the top 1/3 of your class with above average USMLE you more likely than not wouldn't match. The exception to this might be if you are at a top 10 or 15 school and had LOR's from someone famous and did very well in your radiology rotation.
Neurosurg you would definitely need to have honors in your neurosurgical rotation, good LOR's from neurosurgeons and a good USMLE score. There are few spots so "who you know" is probably important, as is the surgeons perception of how smart you are. I think it would be quite hard to get a spot without being in the top 1/3 of your class, but again if you were at a very well known school, might sneak in with a slightly lower class rank if you are well connected and had a high Step 1 score.
People who don't match have to either scramble or do a year of research, if they can find a lab that will take them. They could then apply again the next year, but mostly people just go into their backup field(s).
There was a resident in my internal medicine residency who did an entire medicine residency and then went back and did a radiology one. It's rare but it does happen. I don't know the back story (i.e. whether he originally applied to radiology and didn't get in).
Agree w/above post that usually the medical schools will advice/tell you whether you are competitive vs. not. If they tell you you are not, usually you are not. If you aren't competitive it makes sense to either change fields or at least apply to a backup specialty in addition to derm or radiology, etc.
