[mikeash]

I think there are at least three issues here.

One is efivarfs making it too easy to destroy stuff. It should probably identify problematic hardware and prohibit modifications that break that hardware. This is no fun, but hardware-specific workarounds are a fact of life for real-world OSes.

One is, as you say, bad default behavior with rm. Crossing filesystems by default is pretty weird. Preventing that unless it's explicitly requested would largely fix this problem and many others too.

And one is mounting efivarfs read-write by default. It's too dangerous to be that easy to modify.

I don't think it's too useful to try to pin one of these down as "the issue." They're all worth fixing.