Depending on your wiring, the performance can be terrible.

I have one of these, advertised as 2000Mb/s but actually doing about 80, and in a small apartment. Every 6 months or so it connects to my neighbour's system without me doing anything.

Yes, in principle. In practice I found all the TP-Link ethernet-over-power equipment I used to use to me even less reliable than the WiFi I was trying to supplement. It would work fine for a while, then just lose sync, and I'd have to go round unplugging and replugging everything to get it to sync again. In the end I ran cat-6, but it took me a long time to conclude it was worth the effort. Now my only regret is I didn't decide to do it 10 years earlier.