First of all, this is not a QQ-nerf-it-Thread after i've been beaten by a vampire in Cyrodiil, in fact, i know how to deal with clouding swarm (root them with talons or encase and wait for a few seconds).
After having the skill unlocked on my secondary character i finally realized what all the fuss is about.
The spells you cast in clouding-swarm-form are somewhat muted, so you don't hear the particular spell but how are we supposed to tell what's flying at us if we can't even hear it?!
And there are some spells that are hardly visible anyways, Sap Essence for example or any Impulse that is not fire-based.
The worst thing about it though is that you can't tell an enemy-batswarm from a friendly one until you realize those bats have been sucking on you for 2 seconds already and you've eaten 2 fire-rings.
So, for balance's sake in PVP, give this skill a red circle on the ground like you have given it to every other enemy-aoe, even heals, this is the least that can be done!