this is the part that alarms me the most:
> It syncs the filesystem fully, including Btrfs metadata (which systemd skips).
It suggests that there is something Btrfs is doing—some special IOCTL interface or something—which is non-standard. The filesystem driver should be completely in charge of making sure its on-disk state is consistent across power events without special userland knowledge.
If it can’t do that, it isn’t taking advantage of the fact that it’s in the kernel!
cont