I recently used reveal for a conference talk, and structured it so that most of the slides are vertical. My talk had 5 horizontal slides: hello, topic 1, topic 2, topic 3, and then goodbye. I started off by saying "hi, i'm gonna talk about these three things:" and then scrolling right, then going back left and saying "Okay, topic 1" and then down.

Someone who didn't see the talk but looked at the slides found it VERY confusing.