So far it seems that the Dark Abborition and Crystal Mother were giving me the most issues on hard. Though, I've had issues finding the gap on the Dark Abborition on both normal and hard, the split point between the wide shots seems awfully close to the range on the rail gun, so I have trouble closing to damage it. Not sure if I just need more practice or it should be narrower, but that's my feedback on the bosses so far that killed me a few times.
The Centerfuge and Battlesomething seemed fine. Harder than normal, but definitely reasonable.