Even in hand drawn animation, humans are widely considered to be the most difficult to execute, because everybody has a feeling for how they move.
Brad BirdI think if you have a really big, heavy person, there's a feeling of an invisible puppeteer jerking them around in space. They don't feel like they are moving themselves.
Brad BirdWhen you assemble animation teams the way you do a live-action film, you're often struggling a bit to get a cohesive team together, so if you have a team that works well together, you're hoping for another film so that you can refine the team.
Brad Bird