What they say and what it means:
"It's hard, but doable" = "I've already done it, and I've been keeping it in my back pocket until I can get the most kudos for it"
"It's non-trivial" = "It's really hard, and I don't immediately see a way to approach it"
"It's distinctly non-trivial" = "It may require violating the laws of physics"
"It simply can't be done" = "I don't want to do it or talk about it. Since you have no way to know whether I'm telling the truth, this ends the discussion."