[Answer]-Is using Django MondgoDB Engine considered mainstream

When it comes to more widely and used I personally do not think so.

There is Flask, but Django really does everything it does plus a little bit more.

What is the problem with mainstream if it is proven to work?

I would strongly recommend using Django, specially if you interact a lot with databases. It’s migration system is the best out there.

If your project is rather small and does not need all those nice features I would recomend Flask instead.