AFAIK these are always firing 100+ rounds at every single target and only have ~1500 rounds in them, so that's not exactly a lot of targets.

And then there is the matter that a CIWS/C-RAM would need at least some kind of upgrade to be able to engage (small, slow) drones: https://www.quora.com/Why-cant-we-rely-on-the-Phalanx-CIWS-t...

(I have no experience at all with these besides seeing some probably-fake Youtube videos supposed to show these shoot down mortar sheels)

Likely but than there's nothing left for the next wave. Cheap drones could quickly use up all the rounds. You wouldn't even have to use advanced drones with weapons. CIWS have to kill every drone just in case. Add a few missiles and or better drones in wave two and you got a very effective mix.