Data infrastructure in the real world, for non-trivial systems, moves slowly. Migrations are costly and risky. If you need to optimize query performance, you are likely working on one of those non-trivial systems. We have some MongoDB clusters at work (one in my team). Would we have picked it today? Maybe not, because the mindshare isn't there. Does it work fine right now though? Yes, definitely, we have no issues really (the suck in that system is elsewhere).