Speaking with an "American" accent should be difficult for British or Australians (or other non-native speakers) but it doesn't seem to be. The US is a big country, and there are a LOT of regional and even in some cases community accents -- in some cases, even Americans don't understand each other's accents. Then you throw in ethnic accents, and you have a bigger problem. Then throw in colloquial phrases and words, which again can vary greatly from place to place, and you have a bigger mess. American also love to borrow words from other languages (usually mispronouncing them, again in different ways from place to place), and you have an even BIGGER mess. My mother told me that learning what she still considers weird grammar, weird spelling, and even weirder colloquial phrases and sayings was the hardest part of learning American English -- and she still, 50 years after becoming an American citizen, has problems sometimes with idioms and spelling some words.
An yet, British and Australians
still manage to pull off American accents better than Americans can pull off British and Australian accents!
