upsetting the neighbours
There is another drawback to this technology. It uses typically the entire short wave spectrum (2 - 30MHz), and some of the energy pumped into the house mains wiring leaks out to the surrounding area. This is due to problems with the mains wiring - it simply isn't suitable for the job - and manufacturers are just exploiting this medium because its there and costs them nothing.
There are quite a lot of licensed radio users also using that radio spectrum - such as broadcasting, amateur radio, aeronautical comms, CB radio to mention a few. If you happen to have a short wave user within about 500m of you when run one of these powerline networks, its quite likely you will be subject to a complaint to Ofcom about radio interference. And you will be required to stop using the product because licensed radio users have a legal right to unpolluted spectrum.
You may say that your PLT product has passed all the required approvals for EMC and like. Unfortunately, any such approvals claimed on the packaging and in the manuals, has been obtained by a technical loophole in the approvals system, which the manufacturers are currently exploiting.
BT are now discovering to their cost that they cannot run powerline networking with their BT Vision product when there is a SW user in the vicinity - it has to be replaced with either networking cable or Wi-Fi (both of which work perfectly well without causing this problem).
If anyone buys one of these products off the shelf, I would recommend demanding a written assurance from the supplier that should complaints of interference arise, then they will provide a full refund.