So what is your order of competitiveness?
I have found IR and Mammo the most competitive ones.
Neuro is not competitive at all as the job market is not good for neuro.
MSK is in the middle. Peds is not competitive at all.
Body is getting more competitive.
Cardiac, Nucs and Chest are open doors for almost everybody. You just need to apply to get these.
This is at least my experience. On the other hand, I agree that there is not any official report as the fellowship application in radiology is very disorganized.
Also there is huge regional differences and also huge year to year variability.
So this year MSK may be super-competitiveness in South Cali and Neuro may be competetive in Boston, while in texas mammo becomes competitive. Most people do fellowship in the same region they do residency and most people stay in their home programs for fellowship as they think one year is not worth moving.
In general Radiology fellowships are not competitive. Even for IR you can match somewhere for sure and if you are from a reputable program you will end up in a good one if you do not stay at your program.
I agree with you that this is a very difficult thing to judge.
The best "evidence" that I have that breast imaging is not competitive are the job listings. In an otherwise stagnant job market, you'll still see plenty of public opening for mammographers. Most other job openings aren't even advertised in this sort of market because there are people lining up to take them.
Why is that? I submit it's because most radiologists have not been and continue not to be interested in being breast imagers. Sure, there are a few people who will do women's imaging in a down market just to get a job, but most people are unwilling to pigeon-hold themselves into breast imaging over a 30 year career, even if it means not getting a job in the short-term. The truth is that - even in a down market - radiology continues to not produce enough breast imagers to meet demand.
I also agree with you that, overall, radiology fellowships are not competitive. Like with most things in medicine, the top programs in most subspecialties will be very competitive. But if you want subspecialty X, you'll get it, albeit not necessarily at a "name" program. Accordingly, I think it's more instructive to look at the number of "name" programs for a subspecialty. The less competitive will have fewer (e.g. peds: CHOP, Cleveland Clinic, MGH), whereas the more competitive ones will have more (e.g. MSK: UW, Stoller, UCSF, UCSD, Mayo FL, HSS, Duke, UVA, AIRP).
I also agree that there is a tremendous amount of regional and year-to-year variability, and it may very well be the case that where you are mammo is highly competitive. I just have a hard time universalizing that since it doesn't jive with A) my personal experience from very different parts of the country and B) the job market.