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bcachefs is specifically trying to avoid the btrfs garbage fire by building slowly on top of well-tested bcache. I am confident it won't end up like btrfs where RAID5/6 is still horribly broken yet people claim the filesystem is totally stable and fine for production use.

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You can use btrfs on RAID5/6 without problems if you don't use the RAID features that btrfs provides.

the trouble with block based raid5/6 is the write hole - when you're partially updating a stripe, you can't avoid having the p/q blocks be momentarily inconsistent because there's no way of doing atomic writes to multiple drives simultaneously (short of full data journalling). That means data unrelated to a given write has inconsistent p/q while that write is in flight.

the appeal of doing raid5/6 within a COW filesystem is that it's potentially an opportunity to get rid of the write hole. Of course, the devil is in the details.

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