I think the issue is that with the bad drives, an array rebuild will never finish. One drive will degrade to the point where it simply stops responding.

I understood that to be mostly a problem with hardware controllers, those tend to be more picky? (someone I knew tried a hardware RAID of WD Greens back in the day, that also exploded regularly because of response times)

I'd thought it would just slow down a lot when software tries continuous writing, and am curious how much that is. (ideally, this is something WD should publish, since they claim it's fine, but ...)