94-98's are notorious for blowing HGs. It was something to do with the iron block and aluminum heads (different rates of expansion under heat) combined with a weakly designed gasket. Ford recalled and repaired many of these, but a lot slipped through. By the New Edge years 99-04, they had the problem fixed.
Edit: As for the point of Cobra's being more reliable, they are hand built by SVT. I believe that has a lot to do with the perceived reliability.