I used to work as ISP tech support. If you'd call your ISP and ask for assistance they'd tell you that there's no way in hell they'd troubleshoot while you're connected trough homeplugs (internet trough the powergrid).
All though, I've heard ppl say they work fine. But only for those that use them for basic stuff, such as youtube and news.
Also, if your cables in the walls are older then 10 years, it's never gonna work well.
Personally I used to share internet with my neighbor, the way we sorted it out was to have 2x 5.1 ghz repeaters that were wifi bridges designed to send TV. One was connected with an ethernet cable to the router and transmitted to the other one down in my apartment (2 floors down with one of the floors being concrete).
This meant it didn't get disturbed by anyone using the routers wifi and the signal had only one device to send info to. A wifi will swap between sending to different devices otherwise and usually cause some issues while ex. gaming. On top of that I had a router in bridged mode connected to the repeater in my apartment so I could get my own wifi for phones and such + the extra ethernet ports for computers.
As for your question about what speed that matters, it's all about where the bottleneck is. Your speed will always be equal to the lowest speed on your way to the server, be it your ISPs throttle, your wifi speed or whatever. So if you have 500 mbit/sec to the router but your homeplug can only handle 50 mbit/sec you're limited to 50.
From the problems I've seen using homeplugs, the biggest issue seems to be latency (delay) and/or packet loss due to less then good quality powerlines. But as Iong as your wiring in your house is new, they SHOULD work pretty fine, expect extra delay though compared to wifi/ethernet cable.
Edited by Vicin, 09 March 2018 - 08:28 PM.