I hesitate to generalize too much about ducks and temperatures - I was following "the book" our first winter, and I am glad I closed the book, watched the Runners and made my own decisions.
What I found with my flock is that we have not had trouble with temperature differential between the night pen (no lower than 40F) and the outdoors. I have the luxury of watching them while they are outdoors. So over the years I have learned how to gauge how well they are doing, and I bring them into the shelter if it's going to be too cold. I have an algorithm of how long they will be outside based on cloud cover, temperature, and precipitation.
The Buffs are more winter-hardy than the Runners, by the way. But you can see both groups do much better in sunshine than shade when it's freezing or below.
Just wanted to toss in that some folks gave me the dickens about temperature differences, and we have seen no health problems. But I think the disagreements arise in part because we don't spell out some of the details. My ducks are not going from 75F into 10F or vice versa. The shelter cools off as the outdoors cools off.
I was also told my ducks would huddle together to keep warm. For this flock, that does not happen. No matter how cold they are. I have not had them outside in less than 15F, so I suppose we could say there might be a temperature that would make them huddle.
But I don't think so. Silly duckies.