Yona does have a lot of diehard fans, but I don't think she's a scam. I think she works really well for a lot of people, but not necessarily for all.
I have only had a lenormand email reading with her, and none of the predictions have come true from it so far. However, people on other boards have indicted that her timing can be off by a year or more, and often it manifests as something other than what they thought.
With my reading, what she said sounded plausible and would HAVE to happen in the way she said (if it does happen).