Can we confidently say that all tasks in the queue will be processed by a single thread without exceptions?

In iOS, if we are talking about a serial DispatchQueue, we can confidently say that all tasks in this queue will be executed sequentially on the same thread, without parallel execution. However, the thread on which tasks are executed is not necessarily always the same — the system can change the thread between tasks, but tasks will not be executed simultaneously.

If the queue is concurrent, then tasks can be executed simultaneously on different threads.

Exceptions may occur if additional threads are created inside tasks or other multithreading mechanisms are used. But within the serial DispatchQueue itself, tasks are executed sequentially on one thread at a time.