Being fully honest and direct at work is hard. To varying degrees, most of us want to be liked by our colleagues and employees. Being seen as too tough can feel like you are giving people reason to not like you.
Direct feedback is a critical skillset to learn. Telling people what you need and what you have observed does not always come naturally. It is easy to come off as abrupt, rude, or uncaring even though you are simply trying to be constructive. Depending on your relationship to the person you are talking to, the tone may need to change further to ensure the line of respect is maintained.
Much like dealing with corporate politics is simply part of the job, ensuring objectives are set and met is critical to success. Being able to outline the facts of a situation (good and bad) objectively is a fundamental requirement for success.
Some people take this concept too far and think they always need to be brutally honest. The difference between honest and brutal honesty lies in whether you are addressing just the facts or whether you are drifting into personal territory. Ranting and yelling is the opposite of having a hard conversation. Anyone can yell but few can always keep their cool.