SQL support is on the roadmap [1], along with other great features [2]. By the way, it is also possible to use Meteor mostly for the backend and use a framework like React or Angular for the frontend, although I don't know if there are big advantages with such a setup.
I think Meteor is a great concept because of their seven principles [3], which I haven't seen in any other framework. I haven't worked with Meteor yet but will start a project in short term which uses Meteor, so I'm very excited to see how this works out.