The downside is that monkey patching the world with gevent was hard to pull off and even harder to verify. I was running a large Django installation on gunicorn+gevent for some years and I saw almost no difference in throughput or response time when I made the switch to sync workers. I was glad once I could remove all the monkey patches which really did nothing for my use case.

To this day I am not sure how anyone is getting better performance from using async over sync for something like Django. If you run 1000 sync gunicorn processes, I'm sure the OS scheduler is doing a better job in assigning them CPU time slices than what you could ever do with gevent.

There is a place for gevent/asyncio, for example if you want to fire off some http requests or DB calls in parallel but you'll have to do it manually no matter what gunicorn worker you run on.