[anonacct38]

The author touches on the reason, but I'd phrase it more strongly.

Shared mutable data concurrency is hard because it removes ability to locally reason about code.

    x=1
    x++
    assert(x==2)

can now fail.

[dragontamer]

The x++ isn't necessary.

    x = 1;
    assert(x==1); // May fail in a parallel world