I am a GFN subscriber (and Stadia prior to that) so I am quite interested in this - GFN is great (I no longer have a gaming laptop and just use game streaming -90% from GFN and the rest via moonlight from a server in my loft/attic) but the major drawback is that you cannot access your entire game library. I presume it is a combination of a) compatibility but probably more likely b) contracts/business/legal. GFN does…
It's legal. They used to cover pretty much all of it then published came at them one after another to want a "deal". The moment they gain any kind of popularity, they're going to hit the same legal wall. As for using it for your own needs 1:1, I wish steam would expand it's steam to steam self streaming to cover usage out of local lan.
I do this all the time with my powerful work computer at the office while I'm at home. It works surprisingly well, actually.
The only shitty part is that they removed the ability for you to download a new game on the remote machine from your local steam client. The only way to do it now is with the steam app on your phone.