Transactionality is enough for most systems. Your order either succeeds or fails, it never stays in an incomplete state. Queues are not a panacea, and introduce their own problems like obscuring problems by delaying them long enough to bring everything to a halt.
Yes, you wouldn't do all work in the task queue - commonly, we make some change in the database which can happen pretty fast, and after that transaction commits, we might defer a task that sends a notification, email, whatever.
