I had Mongo lose data.<p>The box it was hosted on was shut down (possibly uncleanly), and when it came back up, mongod refused to start. I ran mongod --repair and ended up with a running system, but the data was mangled. There were parts of records inside other records and a large amount of data that I just couldn't find. Fortunately, most of it was backed up elsewhere.<p>I've been told that maybe some other version is more reliable. I've been told I need to configure it properly by hand before I use it. I don't care. This was a trivial single-machine installation and there is <i>one</i> thing any sane database absolutely <i>must</i> do: avoid completely losing all the data that has been stored in it.
I don't understand how writing migrations takes "weeks (if not months)". I can see "hours (if not days)". But, you still have a schema and migrations, they just exist in the application layer instead without a clear upgrade path.