Btrfs has a "happy path" so long as you don't use any features outside of the happy path, your data will generally be fine. Outside of that, your data is less reliably fine.
Btrfs also has issues with large numbers of snapshots, you have to cull them occasionally or things begin to slow down, bcachefs does not.
Doesn't btrfs fit that description? I know there are some problems with it, but it is definitely a native COW filesystem, abd AFAIK it is "modern".