That is nonsense! Whether you learn English from an American or British English coursebook, 99.9% of the grammar is exactly the same. There are a few differences in vocabulary, but overall these differences make up a tiny proportion of the total vocabulary of the language. If you learn American English you'll be taught the phrase 'mail a letter' and if you learn British English you'll be taught 'post a letter'. But is either one of these phrases actually 'harder' or 'easier' to learn than the other? I don't think so. The written language is the same on both sides of the Atlantic, apart from a few tiny and unimportant spelling differences. Is it easier to write 'center' than 'centre'? I suppose it's quicker to write 'color' than 'colour', but that really doesn't matter too much. You can spell these words either way, and they'll still be understood. The main difference between the two is the pronunciation. Yes, of course there are 'difficult' accents on both sides of the Atlantic. Teenagers on the streets of Liverpool or the streets of Chicago are equally difficult to understand. But nobody is going to teach you these regional accents, and you probably will never need to understand them. What you will learn is standard neutral American or standard neutral British English, both of which are equally useful, equally clear, and - I'm sorry to say - equally hard or easy.

I think they're all very similar. Although, I think some British people have a thick accent. I'm from the U.S. and, generally, I understand other English speakers just fine, whether British or Australian, etc.

I wouldn't agree. There are far more similarities than there are differences between Australian, British and American English. I'm curious to know, why do people say British/Australia is more difficult than American English?