This is such a good children's book. The pirate characters are just brilliantly drawn, and their dialogue is just great - over the top and salted with pirate slang. Even the hero Jack Hawkins is interesting for a victorian child protagonist and he gets a lot of physical action and adventure - which in reality would be way beyond his age and ability - this must be the prototype for all children's adventure books that followed. Not only is the characterisation great but it is also brilliantly plotted and I am sure if you put it against the Hero's Journey model it would hit all the beats.
I think, from memory of the movie versions (I recently saw treasure planet the animated version,) the plotting in the book seems more sophisticated, which is surprising too. It seems like they also stole whole chunks of its plot and characterisation for the original Pirates of the Caribbean movie. Altogether the archetype of children's and YA adventure and I think it would stand up well to any modern competitors.