Cost. Prepregs are pricey. Even when you add up the cost of the and fabric, prepregs still cost more.
Shelf life. This is less of a problem since Fibre Carbon prepregs can be stored up to a year at room temperature. Nonetheless, heat cures prepreg and storage at warmer temperatures will reduce the shelf life. Keeping the material cooler will help and freezing will extend the life significantly.
Necessary heat cure. There will have to be a heat source and vacuum bagging at a minimum. You must be able to achieve a minimum of 270°F and sustain that temperature for a minimum of four hours. Many advanced fabricators use autoclaves but any source of heat will do.