Synology deploys btrfs on top of md-raid. They do not trust btrfs's raid layer.

Because there are some write holes around iirc power failures and, more importantly, restore basically doesn't work (the code just doesn't exist in a working variant).

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.)