The best satire is always convincing to its targets, because it doesn't misrepresent their positions. The Prince may be satire; who knows what was in Machiavelli's head.

Doesn't the guy have another book - The Discourses on Livy, that confirms the general gist of The Prince? (i.e. autocracies are horrible, to be a successful autocrat you need to be brutal and ruthless)

Exactly. Even today, a lot of satire aimed at the 'right', viewed from the 'right's perspective is not realized as satire and is viewed like someone is trying to make a real point. They can't tell it is satire.