I use mongodb only when i have to write an app in Node. Not because it's the better solution , but because mongoose is the only library that deals with data that is mature on npm. The rest is beta at best and drivers for RDBMS are not mature enough.
The mysql driver on NPM is indeed mature. Been using it with out issue for a couple years now in a large app. https://npmjs.org/package/mysql
I'll admit that I originally got into it because I didn't really know SQL and I'm still not very talented with it but for example.. Joins in Mongoose? Say there is a comment.. this comment has an author. If that author is type ObjectId when I run a query I can do this:
model.comment.find({_id: <some id>}).populate('author').exec(e, result) { // author will be populated with that author's data instead of just the ID }