Actually, Ammonium Nitrate explosive, which has a very slow rate of detonation (approx. 9000 fps), is superior to higher velocity explosives in certain applications. That's why it is used almost exclusively for cratering and often used for destroying bunkers and buildings.
High explosive, such as C-4 (approx. 26,000 fps), is much more effective in cutting and breaching, but lb. for lb., does not have near the tremendous pushing effect that a slower explosives has.
Whether inferior or superior all depends on the application intended, however, when improvising as the Iraqis have been doing, they are more than likely just using what is readily available.