I don’t think you understand how strong that stun is. Sure, you can burst Rein’s shield down, but why would you when you can just instantly disable it, Earthshatter, then burst the whole team down? And I think you missed the fact that she can just hide behind her Rein and move forward for half a second to shield bash. That’s not enough time to burst her shield down.
Also, I’m fine with melee attacks going through my shield, but ANOTHER SHIELD shouldn’t stun me through my shield. Especially since her shield is smaller and has less health. It makes no sense logically and it’s not fun mechanically.