Ugh. I didn't expect that. So does it make sense to even have /boot on
RAID1?
It makes administration easier by avoiding special case and need to
manually synchronize content of two filesystems. If it were integrated
in package management, having two separate filesystems would actually
be preferable.
I thought the BIOS would detect the failed disk then move to the next
one and retry?
Well, that is orthogonal. Whether BIOS will do it depends on
implementation. Older systems had fixed boot device; newer ones
probably can use next device in a list if first one fails.
In both cases BIOS is not aware of Linux MD, so it does not matter
whether you have single mirrored filesystem or two independent with the
same content.
To have full fault tolerance you would need to use hardware or firmware
(fake) RAID for it. Actually your motherboard has fake RAID (even two
as far as I can believe documentation); although it has its load of
issues (starting with whether Linux and GRUB support it).