I ask, as my brother-in-law just finished a house where he had asked the contractor to put in internet cabling everywhere, and... drum roll... yes the contractor put in coax everywhere. I tried powerline adapters, but could only get ~30-40mbps (he has a 1gbps fiber connection from the ISP) over them.
I would definitely recommend if fishing cat6 is not an option and you would need to open up walls. Unless your BIL has a lot of kids and their friends over who all like to stream different 4k movies from a NAS, I can't imagine any normal user with 1gbps WAN finding 2gbps LAN insufficient.
In my small apartment in Denmark, with new wiring, they are terrible. I get about 30-50Mb/s rather than the advertised 1200Mb/s. I assume the signal has to go back to the distribution board to get between my adapters.