But that's more because no one dared to try and actually fix Btrfs parity raid.
Misaligned incentives causing lack of funding for something as flexible and generally resilient as Btrfs.
This isn't easy code to write, and it's Linux-Kernel-C, far from "comfy" code to write. I assume it's that kind of effect that continues to keep all competent people from fixing Btrfs's RAID 5/6 code. (If I could gather the motivation, I could probably do it, but that's far from fun to do and I'm already dire for motivation-to-code... it needs to be someone who likes that work, ideally sponsored so they don't loose money from working on that project.)