I'm not sure whether I'm right or not. I don't understand what "long a storytelling asset" means, but I assume that the whole first sentence says that the animation which used to make storytelling ease to grasp eventually became a liability. It means that the animation started to be used more for action scenes and less for storytelling itself. What do you think? Do I correctly understand this sentence?
Making matters even worse, the animation, long a storytelling asset, became a liability. After all, Springfield is populated by cartoon characters. Even the most gruesome injury or fatal fall can be shaken off in the next scene, and the audience knows it. All the taut music and sad dialogue in the world can’t make up for the fact that no one is in any real danger.