I think the discussion about AoE-caps will not bring us anywhere so i will just skip it even though would've liked to see this skill uncapped.
Negate Magic and it's morphs have 2 problems.
1. Every CC-immunity will make the affected target able to ignore it, so if you are negated you can just cc-break it and freely cast spells again. If you had Immovable up before you were hit with a negate, it won't even matter, it will have no effect on you whatsoever. (It's possible that the immovable is negated and you have to cast it again i'm not entirely sure, still doesnt make sense, if you're in the field you should'nt be able to cast anything)
2. The affected target can neither swap weapons nor drink any potions within the area of Negate Magic. These things are no spells so why can't anybody do that? It's not like it's magical in any way to sheathe your sword and pull out your bow.