Actually, we do run redis in production, and it has been awesome for us. But, for this particular aspect of our application, we see about 20GB of data growth per week. So, an all-in-RAM db isn't really practical.

Either way, durability wasn't the issue for us. Mongodb has no write concurrency. So, when it got so overloaded with data that queries were taking forever, we had no way to purge older data without blocking all read & write operations for hours.