The Doomsday Clock is a PR piece for a group of politicized scientists. The threat of a nuclear catastrophe is higher now than it was during the Cuban missile crisis? That doesn't ring true.
It seems entirely reasonable that we're in more danger now than at sampled points before/after the Crisis - after all, it proved that the US and the USSR could manage to not blow each other up. There are more nuclear-capable powers today headed by less sensible men.
Also: "Because, you see, the clock is not a gauge to register the ups and downs of the international power struggle; it is intended to reflect basic changes in the level of continuous danger in which mankind lives in the nuclear age, and will continue living, until society adjusts its basic attitudes and institutions to the challenges of science."