Is the pandemic + on the brink expansion easier than the standard game? (no challenges)
With a game like pandemic it is hard to say in this case if it is easier or harder by just adding in the basic parts of the On the Brink expansion. A lot of the games difficulty depends on the randomness in the game itself. Since player roles are random and with this expansion events are random the difficulty becomes even more random.

In general I would say that the game does become easier as there are some powerful  roles and events in the expansion. However it is possible to get a set of roles and events that do not work well together that can make the game harder. It all will come down to luck of the draw in the end.