Never count on the presence of an earth/ground line
Many countries don't have grounds/earths but they DO have ground trip breakers in lieu of.
With the advent of plastic water pipes, creating a chasm between the street water main (which are also increasingly plastic) many grounds are dependent on ground spikes driven into the ground and saturated with copper sulphate, etc.
There is an assumption of continuing moisture to maintain the conductivity - again this might be lost when the area surrounding a ground pole is covered with concrete!
In North America, as well as large chunk of Japan, two phase + ground (120-120/240) is used and although the 240V phase-phase is used for cookers/stoves and laundry driers, as well as central air-con systems, phase-neutral (120V) is used for regular outlets and connected to achieve balanced loads.
Ontario, Canada, had a 'gold' kitchen system whereby a double outlet would share the neutral line but each half of the double outlet would be connected to different phases,
This means that both HomeGrid and HomePlug would need to have sufficient power to reach the the other phase - via a street transformer - or some cross-connection between phases would be needed to enhance transmissions.