I can't speak for other people, but personally, I always speak my mind. I don't care what level the person above me is. I don't do it in a dickish way, but I will say my say. I always feel that is my duty, to let people above me know what the situation on the ground is, because they just don't know. And usually, in a new job, I'll start out small and work my way up to bigger things as I get to know the system and services/products better. And I've never had any kind of weird shit happen to me, like have someone blow up that I contradict them in the slightest way. Especially in knowledge work. Every boss I've ever had has been good with my feedback and has taken it very seriously. Very often they change their decisions with my input. Often I'll say, "No, can't be done, nope" and explain why, unless they know something I don't know which they always let me know if that is the case.
It's all of our jobs to tell our bosses if something can't be done and be very clear about it, or to have them prioritize work, and also give feedback on that as well.
If you have a boss that is a dick and won't listen - time to move jobs.
Resources are finite, even during flush times, companies go out of business due to squandering of resources.