There is a persistent issue using Mongo aggregate with reactive frameworks like Meteor. The $out operator invalidates reactive updates, which are otherwise possible with insert().

What could be different about the collections created aggregate([$out]) than insert()?

See here:
http://stackoverflow.com/questions/31996392/does-mongos-aggregate-with-out-create-invalid-collections

and here:
https://github.com/meteor/meteor/issues/4947

Seems like Mongo's aggregate() when using $out, somehow produces a different collection than an insert() of the same data. I'm only guessing this because I'm getting very different behaviors.

I'm testing this in Meteor, and collections created with aggregate([$out]) are not observable by the app. I'm not sure why this would be the case, since the collection looks fine in the Mongo console.