The only thing I can find is, in the api doc and release notes:
Working with callback gives you the ability to close the subscription before it’s prepared, which you can’t do right now with the current Promise API. Also, if promise is a good idea, we should add it to Meteor and not just Angular Meteor.
What subscription is being referred to here? Some invisible one I don’t know about?
It’s really good to know that Meteor has the best possible design, and therefore any non-Meteor-inspired features that, for instance, make it easy to write idiomatic Angular code will not be supported in the future, since they are necessarily less than optimal.