Just a few observations - powerline datacomms was brought in as a quick and dirty fix in the pre-Cat5 days, when the maximum RS232 cable run one could safely bung in was 50 feet - a humble 15 metres. Beyond this would require expensive line drivers and a comms engineer to put in a dedicated point to point telecomms cable run back in 1985.
As a quick and dirty - Verrint (I think) brought out the Dataline PLA device, allowing a 9600bps serial connection to be temporarily put in... so you could drive a terminal or printer at a remote location (this was in the days of text terminals to minicomputers). Speeds of this ilk would be way below the mains floor - so wouldn't cause an issue with the mains cabling not being balanced etc. Modulating this onto the mains carrier also was slow enough to nto interfere with anyone.
While different current PLA devices have certain frequencies notched - the very NEED for them to be notched proves the main problem with the tech - the fact that they are pushing a modulated signal out onto unbalanced, unshielded cabling.
As the data rates go up , higher frequencies are needed to handle the data volumes (as well as different modulation methods) - to demonstrate this, you could look at the different comms rates on different freq ranges used in naval comms to submarines.
ELF - 5 baud if that - used as a notifier channel.
VLF - slightly faster
HF - Shortwave (up to 30Mhz) - 10Mbps
VHF - 100-200Mbps
Wasn't there an issue with earlier PLAs affecting shortwave comms? That would also impact aviation, police...
Later ones up the speeds - which affect the frequencies they are modulating to.. and with them sending out onto an unbalanced aerial, they can but interfere with other applications that use the scarce radio spectrum. Imagine how awkward it'll be if PLAs stamp all over 2.54GHz or 5GHz Wifi allocations...
I'm afraid the detractors' point stands. The fundamental tech would only work and not cause interference if mains cabling was required to be shielded in the same way feed cables to radio transmission aerials are. The VERY FACT that frequencies have to be notched out is an admission that the tech is electromagnetically noisy...