3 Antoraks can stop 3 capitals dead: Go with LRMs or Cobalts or Kodiaks, numbers will defeat them, unlike the Dunovs.
They can also keep each other alive by juggling phase out hulls, spreading out any possible spikes 3 ways (and in case of missile frigs, negating them completely) for as long as antimatter holds. Same as the Dunovs, really.
3 Sova's could be countered with a Kol, a Dunov, and perhaps Flak, LRM, or even Marza. Tough but possible.
If they can be caught!
The Vasari Battleships will die to equal numbers of Marzas or Desolators.
Or will they? They get some insane shield regen, a passive to disable abilities and drain antimatter with attacks and a hefty weapon fire rate boost. Desolators have nothing if phase missile swarm is disabled/no antimatter to fire with, Marzas have nothing comparable either.
The Dunovs have weaknesses too! Anything that goes through shields (planet sucker, phase missiles) screw them completely. Having to jump drains their antimatter (as Strattegi pointed out) and this whole tactic relies heavily on antimatter. They can still be overwhelmed with enough ships. They can be ion bolted, yadda yadda. They can't actually break through any reasonable defenses..
Reporting balance issues is fine, but you have to consider everything, not just one scenario. Hell, catch those Dunovs in a magnetic cloud, and then watch them drop like flies! Not every way to deal with a situation has to be with brute force and matching fleet point numbers.
And to add, your proposed change would actually destroy their 'value' much more than tweaking with antimatter consumption/cooldown, simply because they would be support ships that can't support themselves. Right now, what do you do when you see a Kol and a Dunov? You kill the Dunov first. If you see a Kol and 2 Dunovs, the target isn't as clear, but you should still go for the Dunovs (Kol can get 2 regens, each Dunov can only get 1). What if Dunovs wouldn't be able to target themselves? Everyone would just pop the Dunovs with spikes ASAP, and you may as well not even bother bringing them because they would never get a chance to support *anything* because they would be helpless to support each other and blown out of the sky first.