Christians, I have a question about people who try to contact the dead?

I have started reading a book called, "Demons In Disguise" As Christians we believe that demons can pretend to be dead people. People who go to Mediums are really making contact with demons. However, it seems that people who go to Mediums or witness a ghost of a "dead person" say that most ghosts are harmless. It's rare a person will have a bad experience going to a Medium to contact the dead. However, when a person dabbles in a Ouija Board then I often hear of bad things happening. It seems that Ouija Boards are much more dangerous than going to a Medium. Why is this? Why are demons more harmful if you contact them through a Ouija Board, but not a Medium? This is bewildering.