We’ve run into various bugs and weird performance gotchas (like the workers prefetch jobs which is terrible if they aren’t all the same size)
I'm sure if I really tried, I can get celery to be very reliable... but I never really got there.
Also for whatever reason I have NEVER been able to get celery to be reliable for its scheduling/cron stuff. It just starts to fail. I use this library for that, which I have never had problems with: https://schedule.readthedocs.io/en/stable/
I've been using Celery for a long time in production now. Nothing crazy and it's a fairly basic set up of "execute job, thanks!" along with a beat server. Over the last 6-7 years an off the top of my head guess would be that there's been at least 5 million jobs processed. It's not huge volume when measured over years but it's been stable.
One server was running for 6 months without being updated. That's a Celery worker process running for ~180 days uninterrupted. It served hundreds of thousands of jobs without maintenance. A lot of these were pretty beefy tasks too like performing HTTP requests that got 400mb XML responses and then parsed them. I didn't even have things like `worker_max_tasks_per_child` set either.