I haven't tried DJC Bair to break shields, but I've tried the other aerials. To me it looks like DJC Uair works the best, the other aerials seem to push the character back more.
Any Ness experts have a say in this?
Using bair is difficult for shield breaking. In order to use just bairs it would probably have to be done against a wall since you would need to hit with the stronger part of the move. It could be possible with TAS skills though without one.
In terms of choosing what aerial to use, in my experience, it depends on the character and the way the shield is. Uair and dair are the easiest as they are quick and do not require any l-canceling. An easy way to shield break is to just hold up or down while DJCing and pressing A.
For short characters, I find dair to be more effective, and uair for taller characters. Nair is also good sometimes since it has a large shield-hitting range and is good when the shield is small. I find fair and bair to be not as effective, but can be used to start a shield break as they have longer horizontal ranges than the other moves. Usually I find doing a combination of a few moves works best depending on which way the shield goes.
There are many possible combinations for Ness shield breaks. Once the shield is low by doing some quick DJCs, any other quick move can break it. Even breaking shields ending with an utilt is possible if the shield is low enough.