> Also most software runs on ARM now and I don't think that has actually happened in practice.

At least in my house, ARM cores outnumber x86 cores by at least four to one. And I'm not even counting the 32-bit ARM cores in embedded devices.

There is a lot of space for memory ordering bugs to manifest in all those devices.

It's definitely a real issue in real code, since the CPU isn't bound by things like function boundaries or alias analysis or pointer validity. For example:

  x = *a;
  if (x) y = *b;

The compiler cannot reorder the load of b before the load of a, because it may not be a valid pointer if x is false. But the CPU is free to speculate long ahead, and if the pointer in b isn't valid, that's fine, the CPU can attempt a speculative load and fail.

It's not particularly common and code that has this issue will probably crash only rarely, but it's not too hard to do.