I've recently been playing at Hard. I just feel that NTF fighters are BETTER. They're just more dangerous. It may have been mission design, but it seems to me that NTF fighters can strafe you far better than Shivvies can...(except Dragons, that is).
The perception might be there, but it simply isn't the case. A wall of Hercs can put out a lot of fire, so something like Feint! Parry! Riposte! the superiority of the enemy fighters is all mission design. Prometheus Rs are better than nearly every Shivan weapon.
On the other hand the actual fighters of the Shivans are far better. The Mara and the Hercules have very comparable damage resistance, but the Mara is more maneuverable than the Perseus. The Mara that you fly in Into The Lions Den is not actually
that much better than the original.
As you increast the difficulty, the damage of enemy weapons approaches their actual values. That's probably why Terran fighters seem superior to you now that you've moved to hard. In insane, taking even a couple of Prom R volleys form a Herc in a Myrmidon is likely to damage you pretty badly (the Myrmidon isn't a bad fighter until Hard). Shivan weapons on the other hand simply don't do as much damage.
There are Shivan fighters like the Basilisk or Aeshema, which are simply terrible, but an Aesteroth, Mara, Dragon, Shiatan, Manticore, and so are are all forces to be reckoned with when you give it half-decent weapons and AI.