Hanzo can't do anything in a shield war, same issue Widow has but she can position around the shields quicker with grapple. If they have Orisa even if you scatter the shield (which is getting rid of your only defensive ability making you a free dive at that point) that's only half of it's HP. The rest of the time you are shooting at it you are effectively doing nothing because a hero like Genji/Tracer can be actively damaging the enemy back line and causing a distraction.

Realistically if they have an Orisa or Rein you want to be positioning yourself above or to the side so that you have some possible pick sight lines because that's where his value as a hero comes from, same with Widow. Shooting at a shield, even if you break it down, you aren't going to kill their tank firing arrows (since you probably wasted scatter to break it) so the enemy support is getting free ult charge and healing it back up no problem.