That's a great point. Boston accents on film are uniformly awful, and on TV they are almost non-existent. I myself have a *moderate* New England accent, as opposed to a hard-core (that's: haaaahd-coahh) Southie barroom accent.
Even on Cheers they never quite got it right. The closest was Nicholas Colasanto ('Coach' Ernie Pantusso), who was born in Providence, RI. John Ratzenberger (Cliff Clavin) tried his best, but he's from the Connecticut suburbs of New York, which pretty much makes him a New Yorker. And Rhea Perlman is from Brooklyn, and it sounded like it.
Robin Williams in Good Will Hunting was pretty bad too.