Is Aristotle right that ethics is not an exact science?

In Book 1, Chapter 3 of the Nicomachean Ethics, Aristotle says that ethics does not enjoy the clearness and precision of other subjects, in part because even positive values may be harmful in some circumstances. Hence, we must "indicate the truth roughly and in outline, and in speaking about things which are only for the most part true and with premises of the same kind to reach conclusions that are no better." Is that all the precision that's possible in ethics? Or does Ayn Rand's ethics provide greater clarity than was possible for Aristotle?