I find straight, clear professionalism to be "nice", especially when dealing with shitty situations. Usually they're made more shitty by people getting personally emotional over what are, frankly, simply business decisions. A lot of that can be avoided by sticking to the facts and the script, making the inevitable a lot simpler for everybody. It's a matter of showing them adult respect and giving it to them straight.
I don't think they're mutually exclusive at all.
nice = I want to help you
professional + nice = I want to eat you and help you?
Now that doesn't work at all.
professional = I have to eat you (not want)
nice = I am sorry (being nice is not only about helping)
professional + nice = I have to eat you and I am sorry.
?