We shouldn't aim for "brutally honest", we should aim for "honest" (meaning telling the truth, but trying our best not to hurt the other party's feelings).
Also, the author seems to assume that the person being brutally honest is always right in his/her criticism. But what if he/she isn't? This is the situation many employees find themselves in: Their boss lacks the qualifications to bring valuable input to the table, but still thinks he/she should be allowed to be "brutally honest" (which in those cases basically means being dumb and a loudmouth at the same time).
Also, the author seems to assume that the person being brutally honest is always right in his/her criticism. But what if he/she isn't? This is the situation many employees find themselves in: Their boss lacks the qualifications to bring valuable input to the table, but still thinks he/she should be allowed to be "brutally honest" (which in those cases basically means being dumb and a loudmouth at the same time).