I think that Meteor is pretty awesome, but it isn't good for a lot of use cases out of the box... If you need really granular and flexible security around your models, the out of the box security isn't (or wasn't last I checked) good enough.
Common case.. GROUP_A users can read/write their own documents, but only read other user's documents in that group... GROUP_B can read/write their own documents as well as all documents owned by users in GROUP_A. This is a pretty typical scenario for management chain permissions, but when I've looked ad Meteor and similar solutions it wasn't an option (or incredibly difficult to implement).
Common case.. GROUP_A users can read/write their own documents, but only read other user's documents in that group... GROUP_B can read/write their own documents as well as all documents owned by users in GROUP_A. This is a pretty typical scenario for management chain permissions, but when I've looked ad Meteor and similar solutions it wasn't an option (or incredibly difficult to implement).