Technology changes though. PVC membrane roofs require less maintenance and last longer than older shingle roofs.
Flat roofs probably did always leaked when they learned this saying -- but today it's not true.
Flat roofs probably did always leaked when they learned this saying -- but today it's not true.
But if flat ones are harder to design right, then they're more likely not to be designed right, so we end up with a larger number of edge cases where they're going to perform worse.