According to the documentation http://www.sagemath.org/doc/reference/sage/symbolic/units.html the unit behave like elements from the symbolic ring, in which x-x will be replaced by 0. I considder this a bug since clearly you can add two elements only if the have the same unit, and the result will again have the same unit. Currently within sage there is not a neat workaround, I guess you just need to use your brain as you did in your "This works" example.
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