Interesting this, Al.
My esteemed, "The Doc" (PhD in Engineering - electronics engineer, Programme Manager, mad as a hatter!) used to build mains transceivers back in the 50s.
Got the idea from my late uncle, also another hugely bright but nutty engineer.
Apparently during WW II, Radio Hams were banned from playing: so they turned to mains transmission instead! Can't keep a good guy down! This was, of course, not needing any license, as it was NOT free radiating RF.
I was involved in one UK project which was pioneered by a major electrical wiring component manufacturer. Intended to offer remote control over most in-house functions using FM digital signalling, using the mains wiring.
Personal safety (even at our more grunty 230V A.C. ) is ensured, by using an isolating transformer. (Induces an identical voltage, but is isolated from the current! No grunt!).
One point: if you transmit beyond YOUR phase, anyone on another, cannot receive your signal. So a person on the other side of the road might be on a different phase and not be able to receive. Bearing in mind that Step-Down Transformers are used by the power company, when they, locally change higher volts, Three Phase, to equally loaded single phaae, for most domestic consumers: except in France!).
I know! I played with FM mains intercoms in the early 80s'. My neighbour (line of site: 25 yards) could not receive, since they were on another phase.
However, I gather that things have improved since then!
Interference can be type-specific screened out. This is one prob. they must solve. Fairly simple, today.