The problem is if your ship is delayed getting back in to port. It can happen because of weather conditions, rough seas etc. The elephant in the room is Covid which could delay disembarkation. There won't be another Ruby, where passengers are rushed off the ship.
My neighbour, who is a pilot, was shocked to see that I had booked an authorised connecting flight from Brisbane to Sydney to join an international flight. When he couldn't change me to an earlier flight, he gave me strategies to catch a taxi between domestic and international airports in lieu of Qantas shuttle. He knew what could go wrong!