According to the Consejo de Seguridad Vial (Cosevi), 55% of the potential drivers fail their road test, this due to a curve that has to driven in reverse. The other reason many fail the road test is as simple as not obeying the road signs.
As to the written exam, the Cosevi figures indicate that a 45% failure rate.
Failing a test is not only expensive but time consuming. To retake the test – written or road – there is a two week waiting period before, and of course the full test fee is payable again.