I wouldn't say it's a horror story, it's just not really for time series "big data". The backend guys have had to muck about with the data a lot to get good performance out of it. There's some optimizations we missed on the sysadmin side too, like sharding the cluster after it got to ~250GB, and now it's many times that. Our Mongo clusters have been running production for well over a year.

We had a 50gb instance initially and it was no problem, then our app started to get a lot more traffic in a short period of time. We had to start sharding on a reasonably large scale. Mongo is a lot harder to maintain when it's big and unpredictable. I know this sounds like a plug, alas it's the truth, we found Object Rocket and now we don't worry about Mongo.