I think I know the reason why `ref` variables on ARC/ORC are considered generally non-lockable. It's because their destructors are called automatically when going out of scope, without the protection of a lock, which can cause undefined behavior when there're other referencers from other threads. The only solution seems to be destroying them manually with a lock hold, which is no more convenient than manual memory management.
Atomic refcounting works, however, because the access to heap memory is protected by atomic operations even when the ref variable goes out of the scope, and thus prevents races.
