I always do self debarkation, but have run into bad luck when taking a cruise line transfer bus to the airport. Depending on the luck of the draw, you could be amongst the first on the transfer bus. They don't leave until the bus is full. On two separate occasions, I sat on the bus for an hour, waiting for it to fill up.
If you're doing self debarkation because of an early flight, it might be best to plan to grab a cab.