One disadvantage is the poor scheduling. Taking a train at 5:00 am isn’t fun. On the other hand, so far at least, you and your luggage don’t have to go through TSA screenings.
You are on to something there. If I want to take the train to visit relatives in Virginia, I've got to be at the station at 4:30 AM. Plus, there are no parking facilities at the station!
Unbelievable.