Right but "actual information" here is "developers over and over fail to nail what ZFS and MDRAID did well decades ago". There is no reason to trust their next try will be "the one"

MDRAID had write hole until 4.4 (2015, https://lwn.net/Articles/665299/) and it had been long awaited back then. ZFS to my knowledge deals with that by variable stripe length that has its own problems but yeah it works. Btrfs changes implementation of the stripe update while preserving the on-disk format (i.e. can't do the same as ZFS without introducing incompatible change), intent log/bitmap have been proposed (which would be the MDRAID approach) but that's another incompatible change. So the 'next one' is at the cost of performance but with same compatibility.

Did well in what context? RAID5? There were enough people with motivation to spend time on it rather than other things for mdadm/zfs. It's not like btrfs devs wouldn't know how to implement it if you paid them to work full time on it.

Is bcachefs years away? My understanding is the critical chain of code is pretty well tested - bcache.

From widespread use, yes. Bcache was already stable at least 7 years ago. Bcachefs it's not even included in upstream kernel yet, much less any distro as an official option. There's only some plan to start submitting it soon™ and it will take months to complete.