Point defense could negate it entirely, particularly if PD is fixed so that it fires more than once in a blue moon. As it moves so slowly GS’s will almost certainly take care of all of them unless you just spam the missiles. But if you’re spamming missiles than regular fast or multi missiles work just fine.
If these special warheads were immune to PD/GS then the limited ammo thing makes sense. However if they’re immune to PD then they have no counter. Just spam a whole bunch of them and watch as the missile swarms plow right through the shields with no resistance.
I do approve of the idea of making the explosions more gratuitous though!
Think of it like Battlestar Galactica (new one, not the old one). Normal regular hits against the hull are relatively minor, most of the bullets just bounce off, or missiles hit but do little impacts. Then you had the nuke that hit the ship during the series premier. Much, much larger explosion, such that most of the ship was obscured by the sheer size of the explosion.
Put in some sort of slow moving, very long reload, but utterly devastating weapon and that could be fun. Even more fun, give it an awesome graphic.
And it does AoE damage.
Even if it impacts against shields and the shields hold, it should damage everything, friend or foe, within a certain radius.
I’d definitely get behind that!