Your iMessage history disappears; Google can't transfer that to your Android phone. They claim to be able to transfer SMS/MMS history, which surprises me: I'm not sure how they accomplish that. I'm sure there's a ton of other user data that they also can't transfer. (Speaking of iMessage, any group chats you were in are now broken.)
Google of course has an interest in telling people that switching is easy and painless. It's not, though.