Hardened Force Fields are not appreciably better than normal FFs. They've got 5000 armor, but 1/4 the health, and since 1/5 of damage bypasses armor, they're only marginally better against normal attacks, and much worse against high-powered or armor piercing attacks.
Disclaimer: I'm not entirely sure of what I explain below. A player with better technical knowledge of the game might prove me wrong.
Technically, with 5,000 armor, everything that doesn't do more than 5,000 impact damage or have the "ignore armor" perk (actually it's 9999999... armor piercing) only do 10% damage to it. In this case, the rate of fire has no influence because the 10% apply to each projectile (contrary to a lower armor rating where each impact loses damage). For the triangle's bombers, they have x6 against FFields but /10 against that armor: they do 60% damage instead of 600%. The HFField has 25% the health of the same non-hardened FField? I will lasts 250% longer (as far as the offending ship qualifies for the 10% damage: no impact > 5,000 and no infinite armor piercing).
Armor, armor piercing and RoF/impact (and how they are distributed across the units) is something I really want to streamline and give a better overall design. I can't wait for AIW2.
Just an example of how poorly designed the game is around that armor/RoF mechanism: standard fighters have the highest RoF of the triangle ships, so it must be the more sensible to armors. But it also have the highest armor-piercing perk of the triangle. And the bombers, their natural prey, have the highest armor of the triangle. If I were to redesign that, I would make 0 armor for fighter and bomber (hp increases to balance) and some to armor on frigates, then no armor piercing at all on the triangle: fighters (high RoF/low impact) would have a better dent against the bombers and even less power against the frigates; bombers (low RoF/high impact) would be less effective against unarmored fighters and more against frigates. That would reinforce the hull type/bonuses triangle instead of dampening it.
(I keep a long list of design and gameplay tweaks for AIW2; armor/RoF is highly ranked. I did some maths and the Armor/RoF/Impact can have a true meaning in that game... if it were better designed.)