I think the problem with btrfs is that it entered the spotlight way to early. With Wayland there was time to work on a lot of the kinks before everyone started seriously switching.
On btrfs a bunch of people switched blindly and then lost data. This caused many to have a bad impression of btrfs. These days it is significantly better but because there was so much fear there is less attention paid to it and it is less widely used.
Yes, that is what I meant with not as convenient.
Your use case is covered by waypipe (which in my tests is much more responsive than X11 forwarding).
I think you are confusing stuff here. Which kernel has an embedded X server?
What part of your use case is not covered by waypipe?