Why does Alphard wear suspenders? Because in the back they make her coat stand away from the small of her back, allowing her to hide a gun there!
The fact that Canaan's synesthesia doesn't work like the real thing makes a lot more sense once it's revealed that she was exposed to a strain of the Ua virus and essentially made to have it in its current form.
When you consider the "gifts" the other borners get, Yunyun may have gotten off easy. Sure, she needs medication to survive, like all of them, and doesn't have any crazy flashy powers but at least she isn't rapidly aging, or trapped in a teenage body forever, or stuck with a voice that kills. Aside from needing medication, and the possibility of having to have two appendectomies somewhere down the line, she's able to have a relatively normal life.