I like the idea of this filesystem quite a bit, but I'm poor so my opinions aren't necessarily relevant. I understand why you would call it bcachefs given it's heritage, but do you think it might benefit from a new name? I know bcache is well-known among the type to try out new filesystems; and the name bcachefs may be confusing. Just an idea.
My main gripe with btrfs is how it quickly becomes excruciatingly slow after some use and using it with VM disks for instance is impossible while keeping the COW feature on (again because it's slow). Does bcachefs have these issues?