From what I can see, you should only actually down a single Serpent in a turn if there are decent ruins on a table which most tables have. Thats assuming each of those Hornets get 2 Pulse Lasers each?. If they have any kind of fortification such as a Skyshield it gets much worse. I think the Duke does give you some options though, letting you deep strike around cover so that the Serpents will only be getting their basic 4+ cover for moving and holo fields.
I do think it should do great against Tau though.
Veq, you are right that the terrain will increase the survivability of the Serpents, especially when there are ruins and LOS blocking terrain. Eldar speed in general is amazing for finding those weird shots that helps mitigate those terrain. However, the main thing I should be glad about is the presence of prescience and guide as Tom said, as they truly help to make my Ravagers that much more reliable in their hits, which affects their glancing and penetrating results. I am confident in bringing down at least two based upon the attrition of pure glances due to the inclusion of the Farseer. But yeah, without the farseer, the odds are exactly as Tom calculated, if not worse.
And yeah, I am very fortunate to have an regular opponents that play with the current top armies (Tau, Eldar, Chaos) with strong and competitive builds to bulk up my experiences.