I reckon the issue is more in replacement than transient data loss: what are you going to do when you can't find a replacement controller card, or it only available at ludicrous prices?

With a proprietary on-disk format you can't exactly hook them up to any random controller and expect it to work: either you find a new one from the same controller family, or your data is gone.

Replacing your RAID controller is already major maintenance, so there's going to be downtime. I wouldn't be opposed to just wiping the drives and restoring from the latest backup. I routinely do this anyway, just to have assurance that my backups are working.

And a risk! I've had this on a premium machine put together specifically for that purpose and when the raid controller died something got upset to the point that even with a new raid controller we could not recover the array. No big deal, it was one of several backups, but still, I did not expect that to happen.