I tried to use Celery for something extremely trivial (granted, 5+ years ago). It was so badly documented and failed to do basic things I would expect from a task queue (like progress reporting) I have no idea why it was and still is popular.
It's entirely likely that we did something wrong and misused celery. But if many people have problems with using a system correctly then it's also something worth considering.
There’s not much software I really dislike but Celery is one.
A nightmare within a nightmare to configure and run.