It's possible that there could be some technical reason behind the one-shotting. Maybe there's much less armor on a Hades, maybe there's more energy flowing through the cannon that explodes when disrupted. Maybe it's both.
Kinda. It's an anti-air/orbital gun that is destroyed by the player in the campaign. It's the first and last time we see it, so I have no idea if this is a reaper that was modified after landing on earth, or if every destroyer has it, or if there are only a few of them in existence. Too many unknowns. Like why it dies when shot with a Cain. Maybe it's a glass cannon?
Ah, that was the one you shoot with the Cain, right.
Well, I think I'll just assume it's not a Destroyer, because after what it took to kill the one on Rannoch, I think my suspension of disbelief would implode if someone confirmed that you one-shot one with the Cain...