The problem with "asking for the brutal truth" is that no one knows what it is.
Users know, in the sense that they embody it. That's one reason it's good to launch quickly: so that users can tell you precisely how far along you are with the idea.
However a specific group of users does not necessarily embody "the truth" either. An example of this is being captive to customers, as in the innovator's dilemma. Listening to the wrong group of users can lead you in the wrong direction. Sometimes you have to select who to listen to - something that users can't tell you.
Of course, who cares if it's "the truth" or not? Users are users, and approximate feedback is better than none, even if it is not precisely "true".
Users know, in the sense that they embody it. That's one reason it's good to launch quickly: so that users can tell you precisely how far along you are with the idea.