> That's not to say that SHiELD, at least as described presently, doesn't have limitations. A turreted laser can only ever engage one target at a time and there is the aforementioned risk of atmospheric disruptions reducing the beam's range and efficacy

So to intercept 20 missiles you'll need 20 expensive "turrets", your enemy knows how many "turrets" you have and can easily flood it with more cheap missiles.

Also remember that target destruction is not instantaneous, this limits the amount of targets per "turret" and the minimum range.