Of course Umbra would be in the category of hidden bosses.
Morrowind Umbra was great. If you weren't good enough, he gutted you.
Oblivion Umbra was too easy...
Some bosses just shouldn't be leveled at all. Is it frustrating when they kill you in one/two hits? No... just shows you how much further you have to go...
Having Amazing (unleveled) bosses which are way better than you keep the game
interesting because it gives you a run for your money... even when you're dispatching regular monsters with ease. It gives the game more dimensions.