Being brutally honest is telling the truth, but with the intention to hurt someone (ex. telling an overweight person that they are a fat slob with the intention of hurting that person). Clearly this is childish and to be discouraged. However, in common usage, this phrase means any feedback that is not praise. This silly phrase prevents people from being honest and giving real feedback that can lead to improvement.
When someone gives me the fake friendly act, it isn’t nice, it is fake. Being fake is a lie and since when is that nice? It is also damn annoying! If you don’t like something I did, don’t pretend it was ok or that you liked it. Be an adult and tell me the truth so maybe I can take action to correct it in the future. That is not being “brutally honest”, it is being a mature adult and giving real feedback that can lead to improvement. People who do this are the ones we value because they can help you see what you cannot, but need to see. When you see your problems and deficiencies, you can take steps to improve, which is impossible without the honest adult feedback commonly called being “brutally honest”. Which kind of person do you want to be, valuable or fake?