Recently it has become popular to advise against the use of threads, instead advocating the use of other asynchronous processing mechanisms, such as promise. The reasons behind this are that the use of threads and the synchronization involved is complex and error-prone. Often one just wants to run a task in parallel and not concern oneself with how the result is obtained.
For simple tasks which would run only briefly, this can certainly make sense. The main advantage of a thread-based implementation will always be that one can fully customize its behavior. With a promise, one sends in a task to run and at the end, one gets the result out of a future instance. This is convenient for simple tasks, but obviously does not cover a lot of situations.
The best approach here is to first learn threads and synchronization mechanisms well, along with their limitations. Only after that does it really make sense to consider whether one wishes to use a promise, packaged_task, or...