I think single spey is pretty spot on.He can override atc's instructions as long as he has the justification and the kahona's to back it up in the aftermath.
I've seen it before in fog where a pilot was cleared for take off when there was an aircraft lost on the airfield. The pilot refused until the said aircraft was found. Atc got angry and lost rag!. Turned out lost aircraft was sat on runway, would have been big boo boo!
In the end they are on the plane and i'm not, I think if they feel that it's not safe then it probably isn't.