I've also experienced this (in addition to overseas), right here at home with student controllers at military airfields. A good portion of them have pretty thick accents that I guess could be classified as ebonics…….one might think this to be the case, but strangely enough I have really never had the same issue with latino or asian accented people for whatever odd reason. I'll politely ask them to repeat themselves as many times as it takes for me to understand. Normally a guy/gal will get the hint and speak up a little with more annunciation. I try not to be a dick, but I literally cannot understand a word that these types are saying without some extra prompting. I think it is a combination of a typical student lack of confidence in what they are saying/directing, and just an inherent difficulty for me understanding that particular speech pattern. When I lived in Mississippi, there were people who might as well have been speaking russian. The funniest part, though I guess no more unreasonable, was when they couldn't understand a thing I was saying. There are some really wild dialects of English in this country to be sure, which again, the Boston native or the really deep south African American (or cajun white guy for that matter) would probably say about me.