Basically the Nara groups are like Serkr. Jump in at optimal positioning, blast them with overwhelming firepower, get out before the enemy can retaliate.
But with the exception of the counter attack on the Atreus, that the sprint drive nullified, the UEF wasn't in any position to use the Naras that effectively.
Narayana = Sniper rifle - They're used to destroy high value components from long range; they don't have advanced jump drives, so they're less able to jump in - shoot - jump out. Instead they need backup or very good aim. Their standard MO is to jump in at maximum range, disarm their targets, and peck them to death. (Presumably while their fighter compliment supplements their point defenses.) EDIT: Narayana's have decent damage output, but it's not the same as being able to instantly take out a Ravana.
Serker = Humvee with ATM launcher - They're used to blow up large capital assets as quickly as possible, they're MO is to jump in, blow stuff up in seconds, and use their powerful point defenses to cover themselves from a counterattack until their jump drives recharge (apparently a minute or two (Collateral Damage). The tech description also calls them escort corvettes, which is what I suppose they're doing in Aristeia; they might seem to suck in this capacity, but remember they would have taken out one Karuna the same instant the Indus and Yangtze entered in beam range and the other 30 seconds afterward, if ECM support hadn't arrived the second it did.
Chimeras and Bellerophons aren't actually weak at all. They have much better anti-fighter defenses than a Narayana, and Bellerophons have the same number of hitpoints as Narayanas, Chimeras only have marginally less (80,000 compared to 90,000). The glass aspect is that their anti-capital firepower is exposed, and fairly easy to destroy.
EDIT:
The Narayana has some much better tactical positions though (anything outside of beam range)