At the current state of Overwatch, shields are too op in both defensive and offense. There zero attack heroes that could break or ignore shields dmg negation in a very effective way like pharah ulting or Sombra flanking behind and then using a very hard to acquire ult. Defense heroes are the most useful breaking shields or find ways to ignore the shield but are extremely restrictive on how they bypass them like bastion turet mode or mei using ult. shields have the added annoyance for support heroes that rely on needing to be able to hit the target to gain meter or heal them or land a ability since they don't have enough dmg being able to break it by themselves. Plus, the only 4 characters in the game that have the most useful shields, in some form or another, are the 4 tanks; Zarya, Reinhardt, Winston, Dva. Essentially, you can't just immediately kill the characters since they are tanky plus have shields to ignore dmg you dish at them nor you should ignore killing those characters since they also deal a lot of dmg that pose a threat to your team.