I'm curious, why is that? I've heard that sentence before but usually from people who want to write their own task system and end up partially implementing what celery implements just worse.
Celery is large and complex now and edge cases always show up at scale but that is usually not a reflection of the platform quality. The custom implementations I've seen are no where near what celery is capable of and can cater to so haven't seen the edge cases yet but that doesn't mean they implemented bug-free code and celery hasn't.
After asking about it, the issue always went towards a hand-wavey "performance". What is your experience on that front?