The levels aren't too hard, though some are a little bit harder.
None of the levels in this tunnel took part in the Angel Missions, but appeared in the Bridge Building sidequest.
The following is a brief description of the story in the U-Tunnel. More information on the story can be found here.
After beating U-10, the cutscene 'Orbits' will begin, featuring the Duplicator and the Runner. The Duplicator whispers to the Runner asking where she put the Planet, and that he promises not to tell anyone. The Runner looks exasperated as she explains for the second time that she didn't steal it. She then leaves to map out another tunnel and tells the Duplicator to "Catch up once you realize that the Planet orbits faster than us."