"RAID1 mostly OK Can get stuck in irreversible read-only mode if only one device is present."

Which is an especially toxic combination with things like:

"Device replace mostly OK"

It's 30 seconds to find the status page if you're actually trying.

This would seem to suggest they are saying it is okay for production use. Otherwise, what do they mean by "safe for general use"

And yet it hasn't been fixed despite being well-known? That tells me to stay away from it more than anything else.

"even if you do trigger it a one-line kernel patch will bypass the overzealous safety check"

I shouldn't need to hackjob my kernel to make a single individual lone drive work. If you can't work with a single hard drive, you have no business trying to work with multiple hard drives.

You don't. You only need the hack if you do a bad job of cleaning up after the loss of the other half of your two-drive mirror. ZFS won't let you transition in-place from RAID to non-RAID at all. Btrfs just requires that you not reboot in the middle of that migration.