This simply isn't true, huge parts of my house would be offline without powerline, and powerline is both faster and more stable than wifi even in the parts that wifi does reach.
As for noise, networking signals are ordered of magnitude away from the audio spectrum so there's no chance any noise (not that I've noticed any noise) would have an impact on the performance of the WiiM.
Further Networking is digital and digital signals are either perfect, or none existent, noise on the line simply cannot affect the quality, unless the noise occurs after the DAC in the analog realm, if you have power related noise there, it's coming from the power supply for your amp, not the powerline adaptor.