In my experience, this is a really subjective topic.
For myself, I grew up rowing the John Day in April, usually at low water, temps in the 40s, upstream wind, and rain. Consequently, 10 miles of flat water in a lovely canyon with great beaches and perfect swimming weather does not phase me even a little. In fact, I've gone out to Heller Bar from China Bar with no motor in a day, at low water.
However, many many people HATE rowing flatwater. Hate fighting wind. Lack the admittedly delicate and rarely needed skill of reading nearly flat water. It has real consequences too, because ending up in a mile long eddy when fighting the wind can cost you twenty minutes, and the water reading clues are subtle and easily erased by wind.
The answer to this question depends a lot more on your group and their proclivities than any objective description of the water.
Float early before the winds, don't expect to make more than 2 mph, and don't commit yourself to death-march mileage on the flats.
then again, most people tell me I'm a sick sick puppy for enjoying flatwater so much. hell, it's just muscle memory and a perfect rhythm for sippin a brew and watching the walls roll by to me....