PLT adapters radiate their signal beyond the consumer unit (fuse box) and out along the mains distribution network, despite manufacturers claims to the contrary.
This renders them susceptible to interference from, and causing interference to, any PLT pairs downstream of the substation on your phase of the mains network.
In additon they are susceptible to any interference generated accidentally, dodgy connections in wall sockets, plugs, extension leads, whatever. Which may only be short range. Anything with a switch-mode power supply (laptops, scanners, mobile phone chargers) can be a cause of enough interference to upset PLT.
It also introduces a security problem. Unlike wifi where they'd have to sit outside your house to break into your system, they can be in a nice warm house anywhere locally.
IF you are the only person in the area with one, a PLT device will outperform wifi, but because wifi signal attentuate with the inverse square rule (double distance = half signal) but PLT device signals are only attenuated by resistive losses, the range at which PLT devices can interfere with one another is significantly greater.