I used to be a network engineer and I am unimpressed with the throughput connectivity of the black discs. So much so I have used wired connections to each to improve the bandwidth back to the router.
The installation of two of the black discs has transformed the WiFi coverage in my home. The meshing works seemlessly and throughput speeds are fine whether devices are connected to the router or to one of the discs. Maybe I'm one of the lucky ones!
I have no complaints. I live in a "long" Bugalow. My router is in the middle, I have one black disc in a bedroom at one end and another black disc in a hallway at the other. Always have great connections.
The one in the bedroom even reaches as far as my man hut which is 25m away at the bottom of the garden and I can stream from my Sky Q, stream from local media server inside the house and even play on my PS4 with absolutely no problems.
I tried a well known make of powerline adapters but my broadband speed progressively reduced. After several visits from BT / Openreach they discovered that the adapters were causing huge noise problems on my mains wiring which were being picked up by their systems. Once removed and the system reset my speed reverted to the maximum that can be expected for my location and the condition of the underground cables (aluminium) and has remained stable ever since. In fact it has actually increased in recent months. I do have Cat6 cabling from the router to the TV box but the WiFi discs have solved all other connectivity problems.