I've heard this complaint before about Dungeons being repetitive, and I find it the most baseless criticism towards the game. I can agree that puzzles are repetitive, but the dungeon design is always different. Like, impressively so. Yes, there are a lot of Barrows, but at least there's a logical reason for them being there. They are Nordic Burial Chambers. Skyrim is populated by Nords. And even then, each Barrow's layout is different, often mixing in templates of other dungeons, such as Caves, Snow Caves, Ice Caves, Mines, and Forts. I mean think about the word "Dungeon", and generally every kind of traditional dungeon is found in Skyrim, and even then there's variation. See in Oblivion they were just obscenely lazy, but in Skyrim, you can walk into a Glacier, and what appear to be a cave opens up to a Ceiling-less chamber full of trees with a watchtower coming out of the center. See it's all in how Bethesda cross-uses Templates and material found in the game. Where as in Oblivion, they used flat templates for sewers, forts, Aeylid Ruins, and caves. Skyrim combines all their dungeon templates in unique ways, while adding stuff foun in the overworld to create a unique dungeon everytime.
And I didn't even touch on Dwemer Dungeons, each one I find to be completely spectacular.