Complex sentence: They make a similar noise to flip-flops slapping against the heel whilst walking, but the disadvantage of flip-flops when worn on water or dirt is that they will flip the dirt or water up the back of the legs.
Split into simpler sentences: They make a similar noise to flip-flops slapping against the heel whilst walking. When worn on water or dirt, flip-flops may flip dirt or water up the back of the legs.