Has there been a rigorous study of ranking the difficulty of ancient Latin authors? You often see (and, from personal experience, feel) the increase in difficult going from Caesar to Tacitus or Vergil to Juvenal, but I'm not aware of any actual study on this topic.
Of course, I know that there will always be some variation among individual learners depending on what textbook they used, what they've read before, what their interests are. Still, though, some features of literary writing are more difficult than others, so I'd be surprised if no one ever tried to compile a list that takes all that into account. It seems like it would be of great use for effective learning.