There is always a chance of a plane being late and people missing their connection, but there is another later flight from SanJuan to the island and if you miss your flight for some reason American will put you on the next flight out which will get you onto the island in the evening. There were alot of these situations during the winter months where lots of planes came into SanJuan and other destinations late because of delays at departure airports, but it will be taken care of so don't worry.