Both the Iditarod and the Susitna are cool races. The weather is the major factor on the final outcome between the cyclists, skiers and runners. We have a lot of ultra wilderness races winter & summer here. It brings some major contenders in from all over the world.
Some years the bikes have to be pushed through deep snow for miles on end due to the outrageous conditions. The skiers seem to think it's unfair to ride a bike when it's hard and flat, and/or the runners may be subjected to a hundred miles of post holing
I've considered riding the Susitna, but haven't tried it yet. I'm usually skiing throughout winter, but this year I may be riding more.