Chaos ships do fine at long or short range. What they don't do well is mixed in with other vessels.
Compare a Lunar to a Carnage:
#1. The Carnage can put FP18 to one side, whilst the Lunar can manage just FP12 equivalent. So without other ships to interfere, the Carnage would defeat the Lunar in a duel, regardless of the range the combat was fought at.
#2. However, the Lunar has a total all-round equivalent firepower of 33, whereas the Carnage only has FP30. So if the Lunar surrounds itself with enemy vessels, it will do more damage than a Carnage in the same situation.
#3. When the enemy is in amongst you, the Lunar has its most vulnerable aspect covered by 6+ armour, which is equivalent to 5+ armour abeam - in other words, it doesn't care that the enemy has penetrated its formation. A Carnage in the same situation is likely to get its nose kicked in.
So you see, it isn't the range that matters too much. It's the risk that at close range the enemy will be better able to exploit superior total firepower and the soft prows of the chaos ships by diving through your formation. If you can prevent the enemy getting into your formation, it doesn't matter what range you're engaging at.