When I was evaluating MongoDb couple of years ago (around the time they were switching to WiredTiger engine), I've found memory leak in their Node.js client on day one, I've submitted a ticket on their Jira and the same time I had a look at other issues they had there. I saw there memory leak after memory leak, memory corruption everywhere, data disappearing without any reason, segfaults etc. After that MongoDB was dropped as a candidate for a DB in project I was working on, we went with Postgres and never regretted it.