It depends in what context. Some are dangerous because they can give off fumes, some because they absorb oxygen, some because they decompose if they get wet, or are exposed to heat, some because they can shift easily in bad weather.
When I was at college training to go to sea (I eventually went as a radio officer because my eyesight failed), we had to study a large book about the properties and stowage of cargo's which had huge tables listing the cargo's and their potential hazards and how they should be stowed. It was worse in the days of general cargo liners as you also had to take into account the need to keep certain cargo's away from each other in case they accidentially came into contact, and multiple ports of discharge. Containerisation has changed all that, but there are still related risks.
Iron ore was one cargo we didn't like as it was very dense, didn't take up much room for the tonnage lifted, made the ship roll badly because of a lower centre of gravity, and if you were not in a ship specifically designed for carrying iron ore, had a danger of shifting in bad weather.
