You can't just throw random data into it and expect it to work. Proper document modeling and understanding write concern solve almost all the problems people have with MongoDB.
It still has a ways to go but so many of the comments here are issues from way back in v2.0.
Regardless, it's still very popular and only gaining popularity. And improving.
This is why I'm not a fan, its "sweet spot" , in my experience, is not very big and not very interesting. It requires a small data set where at least the indexes (but ideally the entire "working set") fits in memory on a single server, in which case it can get pretty good read throughput. But there are a lot of options that perform great in this use case, including just mysql or postgres. Maybe the flexibility of being schema-less is a win for some people, in which case great, I'm glad it's available as an option.
But when the company claims things like it's the best database for every use case, people are bound to use it wrong, get burned, and then hate it. Where I used it it had been the exact wrong choice to make, a data set bigger than ~2x a single server's memory (and growing) and with relatively high write throughput, and it was an absolute nightmare.