Ethernet over Powerline working between apartments. Should I be worried?
Should I be worried that the two apartments are sharing wiring or that their panels are not separating them properly?


Of course not. You're sharing the same supply with half the street. Everyone on your transformer will share the supply. If it is a three-phase supply then it is likely that 1/3 of the users will be on each phase. It's quite likely that your whole building is on the same supply.

If you were not on the same supply you would not be able to transmit data between apartments using the mains wiring.