Well, I am very surprised to see an email going into OpenBSD vs all on the list, and even more surprised to see it's author it a good veteran of OpenBSD. But I guess things change with time ...
Yes, trying multiboot without backups is a real problem calling. Most of the email is pure hype. Not a single OS to these days is able to handle multiboot correctly. Not a single OS to these days is able to handle any disk correctly. And this is not because of the OS only, too damn many standards.
