This has always fascinated me and this article rings pretty true to my observations/experience. My family is from NYC and its immediate surrounding areas (Manhattan, Brooklyn, the Bronx, plus Nassau County Long Island and the Nyack-New City area), going back many generations in some branches, and their accents are all over the place. Growing up in the midwest made me very sensitive to their different accents and the article helps explain why they have always confused me - I always assumed your borough and/or ethnicity determined your accent, but theirs didn't "line up" with that.

For example, my paternal grandfather (from outside the city, in Nyack) had the thick, old-school Al Smith "thoid avenue" accent, while my paternal grandmother (from Manhattan, child of Irish immigrants) had a sort of NYC Irish accent that wasn't very strong to my midwestern ear. My maternal grandmother (from Brooklyn) sounded very much like Bernie, while my maternal grandfather (from Manhattan, then Brooklyn) had a very mild version of the FDR accent. My parents' generation ranges from almost no NY accent at all, to Trump-like "yuge" stuff, to various really thick accents. Back when they were all alive, family reunions sometimes sounded to my midwestern ear like a meeting of characters from a Scorsese movie, a Woody Allen movie, and an old '30's gangster or detective movie.