neighbours can’t access your traffic as the way they work the meter at your house acts as a firewall, so you can’t connect from outside, if you’re in flats then could be an issue (IIRC).
Hmm, I can see you’re not very technical….
A standard mains meter doesn’t have any specific RF filtering, so the signal will propagate to your neighbours, however it is often the case that your immediate neighbour is on a different phase, as they alternate the houses between the three phases to load balance the three phases. Your neighbours, on the same phase, will be able to see your RF signal.
However, if the gadgets use encryption on the physical/network layer, your data will be private.
NB I used to design powerline network systems many years ago when they were state of the art and hold a few patents on the subject…..