OpenBSD, NetBSD, Plan9, Inferno, openVMS
I suspect by “battle-tested” they didn’t mean World War 2.
(/s of course, and trying out the x86 port is high on my TODO list.)
We had machines in two buildings with multiple machines in the cluster. You could upgrade machines in a cluster and not lose any time. I thought DragonflyBSD is trying to head in that direction with storage (HAMMER).