Documentation is a huge barrier—to have your qualifications recognized. It comes in variable forms, in different languages, and it's not consistent from one jurisdiction to the other. You're trying to estimate the baseline qualifications of an individual. They could have 15 years of training and all the points that go with it, but what kind of training did they have? Training as a general surgeon is really great, but if you've only observed some appendectomies and you might have tried a couple, does that qualify you as a proper general surgeon?
The documentation is probably top of my list, and it has been identified in other research as well. Providing some resources to the physicians abroad so that they could provide standardized documentation would help speed up the process of having credentials recognized.