Everything ran fine, until it was time to fail over - and the company had a 'full failover every quarter' commitment. Every single time we went from the 2-node primary site to the 3-node secondary, all he would break loose. Bad writes, lost partitions, it was a new 'thing' every quarter. Mongo, in my limited experience, was a terrible platform and I recommend everyone who is thinking of implementing it to run screaming. It accepts data SUPER fast, but so does /dev/null.
My HTML5 app failed me 3 years ago, I've now been recommending that everyone stay away from HTML5. /s
And this is not even factoring reports of slow progress, in your face bugs, and PITA manageability.
While it us true substantial progress may have occurred in the less than two years since that test, I would need to see some solid evidence of substantial improvement to consider mongoDB.
That's my outlook based on what I have read.
So they write their app and chuck it over the wall to ops, and by then it's a fait accompli, the ops guys know they will look very bad to management if they say "WTF is this?". So they try to make it more-or-less work, then come on HN to share war stories.