The fact that they don't go as deep makes it a different problem, not one that is less bad per-se. Aircraft bombs that sit 10 meters or more underground take an effort to reach, but that also means that they are relatively safe until you go looking for them. Unexploded artillery shells are shallow enough that regular work such as agriculture will turn them up and that means that lots of farmers will be at risk of their lives for a long time to come without an effort to clear their fields. Mines are extremely bad because they are usually hard to detect depending on what they are made of and can be set off by even the lightest vehicles (and even pedestrians).
