I see, haven't seen these balancers yet so I didn't understand the point. Thanks!

is it ? so ... is the inner belt just useless here ?

As you can see on the picture, inserters won't drop items (steel plates here) on the inner lane of the belt in front of them and just stand idly as long as the outer lane is full. Same issue on the other screenshot where they refuse to drop the bullet thingies on the inner lane. I don't think I'm ...