I didn't find DA:O universally hard, but I did find it variably hard.
I played twice on nightmare, once as a dwarf warrior (2-handed weapons), and once as a human mage. As the dwarf, I was constantly dying to mobs - I have no idea how many times i died, but it was a depressingly large amount. It got really really boring, and annoying, and frustrating too.
Then I played as a mage, going spirit and then entropy spells. I used no creation/elemental spells. I made it through the whole game with only a couple of reloads, but for the most part, i didn't die (I managed to kill Flemeth and the high dragon both first time, too, which was pretty surprising). I got the achievement for not dying all the way through the game. I completely overpowered mobs, killed most of the big bosses first time.. It was... completely different to my warrior playthrough.. Rather inexplicably so, really.
So I think it was just variable, depending on class.