From one brand I read this:
HomePlug has expended substantial efforts to ensure that products designed to the HomePlug specification
can meet these requirements. To meet the radiation limits imposed by FCC part 15 rules, the specification
limits the maximum signal level that can be injected on to the power line. Tests conducted by FCC certified
labs in various regions of United States show that the maximum signal levels incorporated in the
specifications result in emissions that are below the part 15 limits. HomePlug also limits its power spectral
density around the amateur-radio bands by inserting 30-dB notches for the HAM bands in 4.5- to 21-MHz
HomePlug frequency range. Due to this notching of HAM bands, only 76 OFDM carriers are usable in
HomePlug products operating in United States. Joint testing conducted by HomePlug and ARRL showed
that, in general, with a moderate separation of the antenna from the structure containing the HomePlug
signal, interference was barely perceptible. HomePlug is also compatible with other low speed powerline
technologies like CEBus, X10 and LonsWorks as they operate in different frequencies.
Full paper
here.
There seems to be some vehement protest about these units but given the thousands in use is intereference a real or theoretically possibole problem?