Btrfs claims to be stable. IMO, it's not.

It's generally fine if you stay on the happy path. It will work for 99% of people. But if you fall off that happy path, bad things might happen and nobody is surprised. In my personal experience, nobody associated with the project seems to trust a btrfs filesystem that fell off the happy path, and they strongly recommend you delete it and start from scratch. I was horrified to discover that they don't trust fsck to actually fix a btrfs filesystem into a canonical state.

BCacheFS had the massive advantage that it knew it was experimental and embraced it. It took measures to keep data integrity despite the chaos, generally seems to be a better design and has a more trustworthy fsck.

It's not that I'd trust BCacheFS, it's still not quite there (even ignoring project management issues). But my trust for Btrfs is just so much lower.

btrfs seems to be a wonky, ill-considered design with ten years of hotfixes. bcachefs seems to be a solid design that is (or has been, it's mostly done) regularly improved where trouble was found. Now it's just fixing basically little coding oversights. In two years, I will trust bcachefs to be a much more reliable filesystem than btrfs.