I asked our Lead Animator, Ryan Denniston, this question when I saw your post.
He told me Winston is the most difficult character to animate. He walks on all fours and has to carry his gun (which is awkward) like a suitcase. We always make sure that hero’s guns are aimed correctly, so keeping the gun orientation presents additional challenges. Also, the transform is very challenging from a technical standpoint and puts a lot of constraints on the animators and character modelers.