So basically, the writer has more of an involvement in the film over writers who write scripts for live action screenplays?
...in a way, but remember, there will still be a director... and the director will always have the say over the writer, about what gets drawn/shown...
It sounds harder, but it'd also probably be more fun
...a bit different, not necessarily harder... if you're a good screenwriter, you can be a good animated feature writer...