Its powerful but good luck reading the source. Its a bit of a tangled over-engineered mess at this point and the reason there are a number of "newer" libraries to try and replace it. I could not recall specifics since it has been maybe a decade since I last used it but it works until it does not and then its incredibly hard to debug.

Its been a decade and maybe its gotten better but that sentiment is why a number of people do not want to use celery. I would also add that it was created in a different mindset of delayed job and I believe there are better patterns these days without as much complexity.