With ARC sharing objects across threads becomes trickier. First of all, in the general case you'll have to do atomic increments and decrements which tend to be fairly expensive, and they'll be sitting right in the middle your core application logic. Secondly, if you're sharing objects amongst threads, you cannot simply do:

  Thread1:
  Obj = Heap->field;
  Heap->field = null;
  // reduced a reference so:
  if (AtomicDecrement(Obj->refcount) == 0) {
    free(Obj);
  }

Since you'll be racing with

  Thread2:
  Obj = Heap->field;
  // stalls, and Thread1 deletes Obj
  AtomicIncrement(Obj->refcount)

The only satisfactory solution to this that I'm aware of is to use hazard pointers, and that is a fairly complex bit of logic. Maybe there's a better solution to this, but I've not come across one.