[klepra]

I felt like author points out as being nice and good are exclusive traits. In most cases, however, they complement.

[Swannie]

That is true, most of the time someone who is "good" (by the authors definition) will also be "nice". But there are many times where being "good", requires explaining to someone that their performance is below expectations, or that their behaviour is not acceptable. This is not "nice" as the author defined it.

(Though based on the common definition of nice, you can certainly be nice about how you would make such an explanation!)