I don't know much about McKibben, but Googling him I see he's a part of 350.org. Metric, a band I like, had 350.org at shows in the lobby a year or so ago. Just stealing the line from his Wikipedia article:

McKibben has been quoted as saying that he personally believes increased use of nuclear power is necessary to reduce carbon emissions, yet he is reluctant to publicly promote nuclear energy because such a position “would split this movement in half”.

I agree with that entirely. If the goal is carbon reductions, nuclear can play an important role at the expense of nuclear waste and risks. But many of the people actively concerned about carbon emissions are similarly concerned about nuclear waste and risks. And my thought on that is holding firm on no nuclear directly results in more coal and natural gas use.

When we were being taught multiplication and division, I remember being shown a film (may have been actual film; this was about 1988) with a skit where someone was hired to cut a 10" piece of candy into three, 3" pieces. And if they wasted any, they'd be fired. In the skit, the factory said they just couldn't find someone to do this simple task. So the employee cut three pieces and ate the leftover 1". I think of that when I see calls for low or no carbon wind and solar energy supplies. While coming from a good place, it's impractical (at today's costs) and ends up with a very flawed system meeting nobody's goals.