What is the danger of using ground as the return path in normal use?
The main safety risk is that you are pulling up the voltage around the grounding rod.
We like to think of the Earth as a big ball of conductor that's all at zero volts.  In reality, is has a resistance.  So if you put a rod into the ground and pass a current through it, then the soil around that rod will be at a higher voltage than the rest of the ground.  It's hard to predict what the voltage gradient is, because it will depend on the nature of the rod, the current you pass and the resistivity of the soil.
This voltage gradient is a risk for humans.  But more so for four-legged livestock, which have a longer distance between their legs, and no insulating shoes.  So the worst likely risk is that you could find livestock dropping dead as they walk past the rod at either end of the connection.