They probably suffer from very similar issues to X10 stuff:

Reliability in larger houses - especially across phases.
Reliability when used with surge protectors.
Security.

But that also means that the same solutions might work:

Phase coupler in the fuse box.
Judicious use of surge protection. (if indeed this does turn out to be an issue).
Powerline filter at the main breaker to stop signals entering and exiting via the main service lines.