I think they are unique. So far they are all on the island, but the enemies are harder to fight, and I am level 43 and had to go from adept to apprentice. There are some better puzzles that rack the brain.
The side quests are more unique, some have small choices in them, some have time delays as to when they are triggered and are scripted as opposed to radiant. Best side quests in the game so far.
The Kaarstag quest was cool. I'm doing one in a dwemer dungeon right now, and the puzzles were cool. Pressing Buttons and seeing what happens then running on gears.