Treasure Island is an adventure novel. Stevenson spends most of the book crafting cool twists and turns for the plot, and goes light on heavy-duty philosophizing and character development. So it's a really fun book to read: fast-paced and suspenseful. That said, we're still giving it a four on the old tough-o-meter, not because of the content, but because of the book's language.
Stevenson is deliberately creating his own pirate-y way of speaking, and some of the words he uses (for example Long John Silver's "affy-davy" (20.37) which is pirate talk for affidavit, meaning promise or oath) are tough to figure out. Mix Stevenson's intentionally weird words with difficult sailing jargon, and you've got a bit of a challenging read. What we've realized is that you can enjoy this book without understanding every single thing that Long John Silver is saying. His salty speech is more about creating an effect of pirate-ness than about word-for-word comprehension.