The drainage will clog and they’ll risk having a leaky roof every spring. And you can’t get up there with a snow shovel because it’s a green roof and you’d scrape off all the dirt and plants.
(Oregon has one semi-famous flat-roofed house that I'm aware of, designed by Frank Lloyd Wright. I suspect FLW hadn't actually ever been to Oregon: it looked like a house that belongs in Arizona or California. Anyways, the eventual owner of the land the house is built on wanted to tear it down and build a mansion, and after much protests they allowed some people to come in and take the house apart and reassemble it near Salem. After putting it back together, they found some of the original contractors who had built the house originally. Supposedly they took a tour of the house and declared that it must have been properly reassembled, because the roof leaks in all the same places.)
https://en.wikipedia.org/wiki/Gordon_House_(Silverton,_Orego...
A lot of people have inground pools here in Quebec. AFAIK it's just a matter of removing some of the water before it freezes, to minimize the effects of expansion. It does tend to generate small cracks, but nothing that can't be patched over with annual maintenance.
(Love your username, btw)
[0] https://www.davos.ch/en/information/portrait-image/storybook...
I wish they had detailed how flat roofs guards against an avalanche? What is the logic/physics at work there?
https://www.webcenterfairbanks.com/2021/04/04/spring-melt-me...