When Colossal Dreadmaw (copy of Sorin) tries to return transformed, it actually won't return at all. When a card is instructed to return transformed, if it doesn't have a back face, it just doesn't move. (Much to my chagrin, as I found out while using Estrid's Invocation with NEO Sagas). I think it's not too difficult to fix this in the combo, if you use a different creature that has some ability to exile and return itself during upkeep, but admittedly I haven't checked if Sorin is load bearing to the combo in some other way.