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.
I have had the opposite experience from you with wayland and btrfs. Recent data loss with btrfs but perfect functionality with Wayland (on KDE and Arch Linux). Moving panels just works. Fractional scaling just works (though i do miss the old method where smaller screens just got supersampled instead of the way they do it now).
The key, probably, is that you’re using KDE - you’re playing “in the box”. I’m sure it works fine in that situation, or under Gnome; the desktops go to great lengths to make sure they work well under Wayland. Things get more dicey if you’re a WM user and are cobbling your environment out of multiple, independent programs.
I believe you about btrfs; enough people have complained about it that I’m convinced I’ve just been exceedingly lucky. I mean, by now I think it’s probably as stable as anything, but it seems like it used to have more issues.