That's a normal LKML conversation. Nowhere do I see actual bugs pointed out, in fact the only testimony by Carl said that bcachefs has been quite stable so far.

This is just about following procedure. There are people who follow procedure and introduce many bugs, and people who don't but write perfect software.

The bcachefs author cautiously marks bcachefs as unstable, which is normal for file systems. The only issue here is that the patch touched other areas, but in the kernel development model Linus is free not to pull it.