Well of course you have hit the nail on the head, it is not very secure, and in the UK because these adaptors are supplied with every BT vision stb they are common and now that the free football is finished on terrestrial via setanta, they are two a penny on
Ebay
, but still sell for reasonable money
_http://shop.
Ebay

.co.uk/?_from=R40&_trksid=p3907.m38.l1311&_nkw=comtrend+p owerline+adapter&_sacat=See-All-Categories
As regards the compression techniques used you would have to speak to Comtrend I suspect.
They are good for internet access somewhere remote where you are still on the same ring mains like a garden shed or garage