I love MongoDB's query language (JS/Node.js developer so the syntax fits my mental model well), but running a production replica set without spending tons of cash is a nightmare. Doubly so if you have any unoptimized queries (it's easy to trick yourself into thinking throwing more hardware at the problem will help). Lord help you if you use a hosted/managed service.
Just fixed a bug on my MongoDB instance last night that, due to a config error w/ self-signed certs (the hostname in the replica set config has to match the CN on the cert), that caused MongoDB to rocket to 400% CPU utilization (3x, 8GB, 4VCPU dedicated boxes on DO) due to a weird election loop in the replica set process. Fixing that and adding a few missing indexes brought it down to ~12% on average. Simple mistakes, sure, but the real-world cost of those mistakes is brutal.
Just fixed a bug on my MongoDB instance last night that, due to a config error w/ self-signed certs (the hostname in the replica set config has to match the CN on the cert), that caused MongoDB to rocket to 400% CPU utilization (3x, 8GB, 4VCPU dedicated boxes on DO) due to a weird election loop in the replica set process. Fixing that and adding a few missing indexes brought it down to ~12% on average. Simple mistakes, sure, but the real-world cost of those mistakes is brutal.