The root of almost all maintenance problems is water. It consumes wood, erodes masonry, corrodes metals, peels paint, expands destructively when it freezes, and permeates everywhere as a vapor. The vapor from bathrooms, especially in recent, tight-sealed construction, tends to make the bathroom a cancer which metastasizes into the rest of the house. Ground moisture tends to rise and crumble the building from the bottom. But despite this extensive catalog of ways water can attack a building, the main problem presented by water is still from rain entering at the top.
Flat roofs are the worst.
They must have a perfect seal to work and perfection is impossible. Worst is that when there is a leak, it is impossible to tell where it is from. The pitched roof sheds water well and provides room for storage or services. Simple roofs are easiest to maintain. Chimneys, dormers, valleys, skylights, etc., invite problems with flashing and make reroofing difficult.