Well, for one thing, the medium ceases to exist when they are channeling. Their soul "goes" somewhere, so the spirit is in control of their body. If you channel a bad or an unpredictable spirit, they could make you do bad things or refuse to leave your body.

One time... well, actually, I guess it was something else. Nevermind...

As long as somebody stronger than the spirit is around to perform the Spirit Severing Technique I'll be just fine!