PLN is a growing area of research and development. I have 2 adapters running successfully. The frequency used lies in the 'short wave' and is the subject of some unsuccessful lobbying by radio hams due to interference on that band.
The latest move is into the VHF spectrum, up to 300Mhz. Here the prospect of interference becomes more of a nationwide issue.
This video, although not particularly scientific, is effective in showing what might happen, and it is of interest that some of the frequencies onto which the technology may move could interfere with 'advanced' broadband distribution, eg VDSL. OfCom have produced a report.