Only that the depth of the beam is the main determinant of its strength. The plywood I-section will have a known stiffness and the span of the beam between supports and the depth of the beam will be matched to reduce deflection to the allowable amount.
Fire resistance shouldn't be an issue unless there is subfloor electrical wiring what's going to ignite it? Unless it's a bushfire most fires start in rooms or the ceiling space.
The floor will resist the fire longer than the walls and ceiling.
If it was up to me I would go with an engineered steel floor support system not timber of any kind.
