I think it is slightly OP, but I think it would be a mistake to overnerf it. If anything, it shines in the current metagame of mainly physical dps combined with stun spells. If you have a power nuker on your team you can often remove her shield in a single nuke, making it much less powerful.
So anyway, nerf it, but don't overnerf it, because if nukers ever really come in vogue it will be totally useless.
And let's not forget that she honestly isn't that powerful in lategame teamfights anyway, so her shield on Yi is her biggest contribution.