Accoding to the status page of Btrfs, it is not very stable as of kernel 5.5, and more updates and fixes are still on their way.
As of writing OpenWrt uses kernel 4.14, but I can see the driver for Btrfs is in a separate package kmod-fs-btrfs. Will that package be updated to reflect changes from newer kernel, or it is a part of the kernel 4.14 itself and won't be updated?