If mongo is just a document database, there's really no reason to use it over elastic. The query story is slightly nicer with mongo, but we're not doing relational algebra here, right? Elastic crushes mongo at literally everything else.
ES had a bug where a syntax error in a query would send it in a weird state that would then give wrong answers to all queries until the process got rebooted. Which of course isn't fast.
I reported the issue, and thankfully I changed job and never had to deal with ES again. After a few years they contacted me to ask if the issue was still there, I said I had no idea.
