I think my main problem (somewhat ignobly) with this book is that none of the characters are very likeable. The only person whom we are unambiguously meant to applaud is the orphan Imogene, and she is very much a minor character. Dr Syn ought to be the hero, or at least the antihero, but the descriptions of him in terror or dead drunk tend to undermine any attempt to romanticise him, whilst the Scarecrow may be cunning, but he is also a coldblooded killer who has no qualms about betraying his own followers and friends for his own convenience - a plausible characterisation for a real-life pirate, but not really the stuff of swashbucklers. Jerry Jerk, who torments his teacher and has a mania for hanging, is not an attractive boy-hero, and Captain Collyer, who is a resourceful man trying to do his duty, is evidently on the 'wrong' side of the plot. It's not clear quite how much the Squire knows, but he takes no decisive action either to support the smugglers or uphold the forces of law and ends up as a bit of a nonentity, whereas his son Denis is a boastful but weak stripling who doesn't particularly appeal at the start of the book and then becomes a mere plot mechanic.
I can imagine this as a swashbuckler film with the roles of Denis and Imogene expanded to fill the positions of juvenile lead, or with Jerry as intrepid child star, and with the Scarecrow as a carefree daredevil mocking at the stodgy forces of the law with the aid of his loyal sidekick. But this book isn't it. Even the hinted supernatural elements (is there really a zombie among the Captain's crew?) aren't really played up for their full potential.
And yet I'm not sure it's really a case of deliberate moral ambiguity either. This is no "Treasure Island", with its beguilingly wicked Long John Silver. It's more a narrative of Dickensian grotesques, minus the humanity that Dickens generally manages to install in them. The plot's a promising one, but it's harder to care about what happens to the characters.