Say each dungeon has a base percentage for drops, the first one being like 10 percent and increases for each dungeon. Its only fair as you need to spend more resources the higher the difficulty. Besides, once you start hitting dungeon 5, you need extremely strong heros and you either are there for experience or slimes. Also instead of the stupid all or nothing, how about a higher chance for small drops and a smaller chance for big drops. The selection screen led me to believe that 0-5 seems like you have a chance to get any amount of shards between those numbers.