1) The height difference: Madison is 5'1" and Evan is 6'2" which would be great in pairs but may be awkward in the close holds of ice dancing. On the flip side, this could make for some impressive lifts, which Madison lacked with the much shorter Zuerlein.
2) Injuries: Is Evan truly healed from his achilles injury? I initially suspected the reason he split from Emily was that he was not yet ready to compete, though this new pairing would suggest that he is.
3) Style: Though Madison is a beautiful skater, she doesn't have the long clean lines of Emily Samuelson, which was one of the things that made her a great match for Evan. On the other side, Madison is incredibly expressive, even to the point of chewing scenery, while Evan is a bit more reserved as a performer.
4) Status: Oddly enough many expected Chock/Zuerlein & Samuelson/Bates to compete for the 3rd slot in the US behind Davis/White & Shibutani/Shibutani. Since the two teams split and re-teamed, if they can mesh well by January, they are almost guaranteed that 3rd spot on the US podium and a bid to Worlds. USA also has 2 host picks left for Skate America and this team will more than likely get one of them.