My biggest gripe with MongoDB is the memory requirements, which did not become immediately obvious to me when I was first using it. If you index a field, Mongo

My biggest gripe with MongoDB is the memory requirements, which did not become immediately obvious to me when I was first using it. If you index a field, Mongo will shove every record from that field into memory, which can add up very quickly, and can catch you off-guard if you're not accounting for it. It is otherwise a fantastic database that I find myself being highly productive with. I just don't always have unlimited memory to waste, so I opt for more memory friendly SQL databases.