Here are some excerpts from the standard firefighting manual, Essentials of Fire Fighting and Fire Department Operations.
New construction frequently uses lightweight truss made from wood [flammable, obviously] held together with small metal plates in the ceiling spaces. When there is a fire, the metal plates can detach from the truss and the ceiling collapses. While this could be made safer by putting sprinklers in the open spaces of the building, code only requires sprinklers in the living spaces. Therefore, when the fire spreads to the open spaces - or if it originates there - the fire moves through the whole building extremely rapidly.
Please read through this firefighting manual material to understand why firefighters are concerned about this new construction.