@Hady_Rashwan Great questions! A lot of these are actually covered in DreamFactory blogs and docs. For performance and scalability I would recommend reading our blog post on scaling and another on performance benchmarks.
With regards to authentication, Dreamfactory provides a handful of methods check out the wiki entry here.
The free tier is only limited to what data sources are available as REST endpoints. With some planning and performance tuning 'free tier' will scale without issue.
Complex queries can be achieved via the API. For understanding this in detail I would recommend reading the wiki here. You can achieve quite a lot through the API alone with regards to complex queries and even schema management.
If DreamFactory doesn't support a database by default we will research the vendor and hand it over to the product team for further discussion. DreamFactory is open source and hosted on GitHub so if you were so inclined you could submit a pull request. You mentioned RethinkDB, is this what you had in mind?
With regards to the advantages of going old school I'm not really sure I can speak to that. To me it seems much more advantageous to standardize on one platform and reduce the complexity of having to manage all the extra code. Not to mention the added productivity and shortened time to market for your application.
For backing up and moving services between clouds/service providers we offer a handy packaging and deployment tool which makes moving between dev,test and production and even clouds/service providers a seamless process. I would check out the wiki tutorial here.
I hope this helps give you some ideas!