I got married on the Grand Princess at sea. It was wonderful and stress free. The Captain performs the wedding on the days the ship is at sea, therefore no stresses of getting off the ship, marriage licences, etc. etc. The wedding coordinator arranges all of it and you just show up.
I didn't want to look back on my wedding and someone say "what was Cozumel like" and me say "I don't know, I just got married there". I wanted to be able to enjoy the ports that we went and not spend the whole day rushing around for the wedding and not see anything else.
I highly recommend the chapel weddings with Princess. The chapel is beautiful and the ceremonies are done well.
Of course there are always small things that go wrong but that is the case with all weddings I am sure.