Main disadvantages:
- price
- flights - price & availability
- more difficult to get excursions and other bookings
- selection of cabins
Main advantages:
- you're going on a cruise
- you don't have to wait too long to go on the cruise
I can't wait until I'm retired when I can randomly check out what's on offer and pick something that's sailing within the next little while. I can work with the disadvantages.