> To clarify, Linux is fault tolerant where it needs to be: keeping userspace faults contained.

Agreed.

> And what happens when a core module that is part of BEAM starts overwriting critical BEAM VM data structures?

Segfaults and other terrible things.

> A more apt comparison would be software running on top of BEAM vs. a userspace process running on top of Linux.

That's a better analogy of course. I've heard of BEAM VM described as an "OS for application code". Nobody would want to put their latest and greatest crown jewel production code on a Windows 3.1 platform, where one segfault in the calculator process takes down the word processor, but that is essentially what is happening when using shared memory and concurrency units (threads, goroutines, co-routines, green threads etc).