I have four quizzes ranked impossible. These are typically wordplay quizzes in brain teasers and music. They do not fair well.
I have 12 very difficult; those vary greatly in rating.
I have never had a simple rating, but tons of very easy.
Simple used to be quite common, and you'd see it in almost every wordplay for kids quiz. Nowadays, I haven't seen one. I for some reason think that I remember once Terry saying he removed simple rating. I am probably just going crazy, but that is what I recall.