I agree with everything up until this point:

If the road were sloping towards drainage at the centre, then floodwater fills the entire road basin before overflowing, and your road is blocked

I’m imagining the scenario of a residential suburban neighborhood, on mostly flat land. With moderate rain and fallen leaves from the Autumn, some storm drains will clog and need manual intervention. If this neighborhood were built with conventional crowning, then the properties unlucky enough to be next to a clogged drain will see some flooding, but other homes in the neighborhood will have no issue.

If instead the entire neighborhood used center gutters with center drains, but spaced the drains closer together so the drains/km is identical to a conventional build, then the same rainfall should cause smaller impacts, because: 1) a single clogged drain will only flood a small patch of the road, until 2) reaching the next open drain, which is closer and thus the flood is a smaller area, and 3) does not disadvantage the unlucky property immediately adjacent to the clog, since the flooding is concentrated at the road center.

In both constructions, the road area is identical, the rainfall is identical, and the storm drain capacity is identical, yet the latter needs only one half the linear gutter distance and can spread out the flood risk across the whole neighborhood. Phrased another way, center gutters should stave off flood damage to any property, until such point that the drainage is simply overloaded and then every property would flood. No more “lucky” or “unlucky” neighbors: either everyone is safe or they all need to evacuate.

That’s the situation in a flat neighborhood, but in a sloped neighborhood, center gutters aren’t any worse: the most critical drains are at the bottom of the neighborhood. If those fail, it’s still game over for those adjacent homes. And that still is the case for side gutters anyway.