Humor is subjective; what I think is funny, someone else might not laugh at. For this reason, I believe humor is more difficult to write successfully. Also, I believe it is much harder to teach someone to write humor than it is to teach them to write drama. For example, if you put a lot of work into developing a likeable character, and then kill them, you're on your way to getting a sad reaction from the audience. But you can put the same amount of work into a joke and nobody finds it funny. That's not to say that getting a sad reaction from your audience is easy, but I do think it is a much different process.