I recently measured the temperature of a dark tile roof, and a white colourbond roof.
The dark was 70 degrees, and the white was 41 degrees.
While reflective foil does limit the heat getting inside the roof, it is rarely installed with ideal circumstances, with the required 25 mm air gap. Much heat still gets through via thermal bridging.
Reflecting heat, before it has a chance to enter the roof space is a far better option than trying to stop absorbed heat from entering a roof space...... which wont happen.
A white roof and well installed reflective foil will massively reduce air conditioning requirements.