Yea, Hardened are better against some of the traditional FF-killers (bombers, bombers, bombers, and more bombers) but substantially less effective against things that have enough damage or AP to ignore armor.
Their point isn't to be better than FFs (though if you're about to take 10,000 bombers to the face they're obviously preferable) but to provide a way of getting more FFs and also have a tactical wrinkle. You may even consider HFFs to be across-the-board-inferior but still be willing to plunk down 500 K to get HFF mkIs earlier than you'd be willing to plunk down whatever it is for FF mkIIs. And once you've unlocked FF IIs and FF IIIs you may want to go for HFF IIs (and possibly IIIs) just to get MOAR HPS covering some critical point.
Anyway, I'm willing to increase HFF health from 1/4 of normal FF health to 1/3, thus making HFFs about 167% as effective vs bombers and 33% as effective vs its counters.
On the plasma-siege thing I'm willing to make science labs immune to the while-under-ff splash, just to avoid annoyance (it doesn't cause much else to lose those). And possibly space docks, though those probably just need to leave remains instead (though then someone will want those to remember their queues until rebuilt, and that would be a pain in the infrastructure). But the overall effect isn't something I want to be simply blockable, HFFs or otherwise. If the AI can get starships in range of your home command station it should be able to hurt you. So the defense is not letting it do that, which is left as an exercise for the reader