I can see this both ways, really. Wait, let me start back at the beginning. I'm thinking about the difference between writing a story that's brand new and continuing a story that's already begun to be told.
Take Silver Sagas for example. Unlike many series with which I am familiar (the Sacketts, Anne of Green Gables, etc.) the fairy tribes have some very fixed attributes that are easy to mix up. It can throw me right out of a creative spurt, for example, if I realize that I've been describing a Plant Fairy with blue eyes. Far worse is what happened when I suddenly understood that I was about to attribute a basic Sky Fairy function to another tribe in Troubled Skies. That threw the whole book off! (I'm still determined to finish it, but it's a perfect example of it being far easier to do something correctly the first time.)
Now, if I'm writing a brand new story, certain aspects of it are much simpler. Had Troubled Skies not been set in Fairydom, I could have finished it by now. Cambrian, our understated hero, would have made huge strides in his personal life, and the mystery of the missing windship The Talon would have been solved about two months ago.
And now to make my argument the other way 'round. If it's a brand new story, there are no points of reference. (I write fantasy, primarily.) So and so is youngish (which boils down to what number, pray tell?), with clear gray eyes and an anti-gravity belt. I don't know where s/he's from, or going, why, when...whew. That can be fun, but it can also be as exhausting as trying to keep all the details of an already constructed world straight.
And there we have it. In some ways one is easier than the other. And vice versa. ;-)