I can't speak with any authority on ZFS, I know its structure the least out of all the major filesystems.

I do a ton of reading through forums gathering user input, and lots of people chime in with stories of lost filesystems. I've seen reports of lost filesystems with ZFS and I want to say I've seen them at around the same frequency of XFS; both are very rare.

My concern with ZFS is that they seem to have taken the same "no traditional fsck" approach as btrfs, favoring entirely online repair. That's obviously where we all want to be, but that's very hard to get right, and it's been my experience that if you prioritize that too much you miss the "disaster recovery" scenarios, and that seems to be what's happened with ZFS; I've read that if your ZFS filesystem is toast you need to send it to a data recovery service.

That's not something I would consider acceptable, fsck ought to be able to do anything a data recovery service would do, and for bcachefs it does.

I know the XFS folks have put a ton of outright paranoia into repair, including full on disaster recovery scenarios. It can't repair in scenarios where bcachefs can - but on the other hand, XFS has tricks that bcachefs doesn't, so I can't call bcachefs unequivocally better; we'd need to wait for more widespread usage and a lot more data.

The lack of traditional 'fsck' is because its operation would be exact same as normal driver operation. The most extreme case involves a very obscure option that lets you explicitly rewind transactions to one you specify, which I've seen used to recover a broken driver upgrade that led to filesystem corruption in ways that most FSCK just barf on, including XFS'

For low-level meddling and recovery, there's a filesystem debugger that understands all parts of ZFS and can help for example identifying previous uberblock that is uncorrupted, or recovering specific data, etc.