I think of Hard stuff as the endgame (~60% completion so far) and Vicious as something to be done when I can do Hard dungeons sleepwalking. That would be alright except for monster classes are hidden in Vicious. So far I have 1 out of 3 and while I'd love to unlock the other two I'm not even trying. The game clearly showed me it's beyond purist Hard Gan-Teleet win level.
Avatar was curious what makes Vicious too much. My experience is very subjective - I have dragon isles done, and 2 attempts on naga city. Didn't even explore other places when I unlocked them.
Dragon Isles are at least open (subdungeons/map accessible once you get to the boss) and have altars available. That makes me think they're doable with most classes, though I'm postponing that.
Naga city - I just plain don't understand it yet. No altars for leveling, locked out of main map, minimal blackspace to regen and lvl10 mobs with nasty afflictions. I don't even see why use gods at that point. They're all crippled by then. Could use shop resources better I suppose. Still - I don't think all races/classes can beat it.
Dunno. Overall Vicious stuff feels like it needs extreme minmax and I don't enjoy that much, so I'm steering clear.
Gold challenges are very different. They're an ultimate test for each class. Some may be TOO hard, but overall I think of them as the second "win" objective in the game (although the game certainly doesn't present them as that ).