Earlier quoted context omitted.
Corrugated iron is infamously loud in rain. What type of metal roof are you considering that wouldn't be such an earsore?
Here in NZ most roofs are corrugated iron, and people love the sound of rain on them. Different cultures... There is another good reason for our love affair with iron - we get a lot of earthquakes. An iron roof provides a lot of structural bracing, and it's negligible weight provides much less moment than slate or tile alternatives.
Wood shingles rot, are slippery when moldy, are an ideal habitat for wasps, burn, and generate a demand for old growth cedar (i.e. not sustainable).