I've used gevent monkey patching for years in real codebases with success although I have not used celery. Most problems I've seen with this come from not doing the monkey patching before importing anything else.

Also, generally avoiding c-extensions for talking to databases unless they have explicit support for greenlets is advisable.

My main webapp currently is Flask on gunicorn with async gevent workers. Mix in Flask-Sockets (using gevent-websocket) and I have websocket support as well and use redis pubsub as a broker to communicate intraprocess. It's a nice and capable system and you can reach into gevent if you want to do scatter-gather sorts of patterns without dealing with threads.