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.
they do not work for individual applications
as someone else replied to you earlier, waypipe exists, and is packaged in distros, and does what you’re asking for.
There is also a newer thing called wprs, “Like xpra, but for Wayland, and written in Rust”: https://github.com/wayland-transpositor/wprs#comparison-to-waypipe which sounds promising