You still have to trigger it when you get caught in the slide (pull cord like thing if I remember correctly). So with both the airbag and avalung, you have to realize you're caught/going to be caught quickly enough to perform that action. The airbag should keep you at or very near the surface, however if you're face is buried, you could still be in trouble if someone doesn't get to you fast.

In the end, (IMO), the first thing is to pay attention to terrain and avoid dangerous situations. Realizing that it is impossible to have 0 avalanche danger in slide prone terrain, you (and the people with you) better carry beacon/shovel/probe, and everybody needs to be johnny on the spot with them.

The airbag and avalung are nice, and will/have save lives, but they are not at all a replacement for the above.