Room to room wouldn't go via a subststion (unless the rooms were on different phases)
However different houses on the same phase is entirely possible - there is nothing in the consumer unit specifically designed to block a home plug signal.
The only way to keep the signal from traversing the consumer unit would be to install a low pass filter in the mains supply.
From the point of view of data security, anything using ssl (internet banking, for example) would be no more vulnerable than an unprotected wi-fi connection, as the data to/from the server will be encrypted. However, unencrypted data could potentially be intercepted.
However,
afaik, home plug data is encrypted over the home plug link, so that provides a degree of additional security, depending on key length and key uniqueness.