The depth alone does not determine whether it is a deep or shallow sand bed. Size of sand particles, depth, and macro-fauna determine the type of sand bed and method needed to maintain.
These are all shallow sand beds, particle sizes and depths
Sugar: up to one inch
Fine: up to three inches
Coarse: up to five inches
Deep sand beds are something different. The macro-fauna circulate water keeping it healthy and in balance. This allows for more depth giving more filtration. Now if the wrong macro-fauna is used or the macro-fauna die so does the tank.
A shallow sand bed is designed so that it will not crash if you keep the detritus from clogging the sand.
In both deep and shallow sand beds it is the areas of water having no oxygen that potentially makes the system crash.