There are both ABI and API breaking differences between MongoDB 2.x and MongoDB 3.x
Both commands (e.g. addUser vs createUser) and behavior (e.g. a function that used to return and empty object in certain cases now return null instead) have changed.
I suspect MDG would have to go through much of the core code to upgrade to MongoDB 3.x to make sure nothing breaks. I’m sure they will, but they probably want to improve developer experience before binary upgrades.
Soooo, you are saying that if I were to be writing something that used those particular commands, I would probably end up in trouble?
Because now I can add users in the accounts package UI and it works fine. I don’t have a need to access the user database directly, other than that. I have extended the user database with another collection that I have easier control over.
No, the example commands I gave are MongoDB commands, and have nothing to do with Meteor. The addUser and createUser commands create MongoDB users, not Meteor users.
Let’s say there Meteor has a meteor auth-aginst-mongo command (which it doesn’t). If this command was using MongoDB’s addUser command in its inner workings, that command would fail if the MongoDB end wasn’t 2.6. This example is of course completely fictitious.
Specific to those MongoDB commands, I don’t know of any package that uses those commands (I don’t think there are any, Meteor does not care about MongoDB authentication), so you are safe in there.