So, MongoDB is recommended here on FCC, since it is based on JS, and it is part of the MEAN stack, but most of the comments on the Internet (StackOverflow, Quora etc.) about MongoDB are very negative… It is almost the same as with PHP - everyone is bashing it. But PHP could get me a job in my country (not USA), and there are almost 0 MongoDB (or entire MEAN stack) jobs on my market. I know that MongoDB and PHP are unrelated, but you get my point - my goal is to get a job, and MongoDB is pretty unpopular, with (for example) Google engineer bashing it as the worst part of the MEAN stack, and recommending people to use PostgreSQL or something else… Also, Google Trends are showing almost a flat line when asked about MongoDB popularity over the past years. All in all, would it be better to learn MySQL or PostgreSQL instead of MongoDB, since there are much much much more jobs available for these database solutions, at least in my region?
Learning the basics of MongoDB is quite trivial compared to SQL (which itself is not super hard either until you get into more complex operations).
Since it leans on your existing knowledge of JSON, I’d wager that it’s easy enough to build a demo app with to try for yourself without much commitment.
The Mongo team suggest that their ‘document’ model scales easier than a relational table model, but I’m not nearly well enough versed in dev ops to speak to that myself.
Give it a whirl; it’s pretty fun 
Well, I guess nothing bad will happen if I devote few days of my spare time and try MongoDB. But I’ll probably try MySQL and/or Postgres, too, and see what suits me better.
More you know, will find the best solution for different problems, you’ve to learn all those and more, but approach one at a time for several months.